What do I do? is my mum cheating? Do I confront her? Do I tell my Dad?

Speak to your mum about it.
Don't bring it up with your dad yet. From what you've said no cheating is occuring (for all you know, your dad knows about it). It sounds like she has discussed her sex life with your dad to another person and sent a photo of yourself.......doesn't necessarily sound like cheating

Tell your dad. It’ll hurt him more if he knows you knew as well. Over a year? As a man I don’t know much about the menopause but even I can see that this is calculated and intentional.

Yeah I agree. But I do think further up it was a bit more. Like him asking her sexual questions and stuff like that. Do I go back and read more, to check or whatttttt

Okay. So basically my mum and dad have had an amazing relationship it would seem. She is soon to be turning 50. I would say we have a very good relationship and are closer than most mums and daughters.

Over the past year or so shes been acting very differently. Spending a lot of time on her phone and being tearful about other situations.
She has even eluded to me that shes falling out of love/fancy for my Dad, but then thinks its so sudden. Suggesting she might be going through the menopause or something.
Around August last year she lost quite a bit of weight maybe 3 stone?

Anyway she plays this weird Words with friends game and I have caught her messaging people on there, but its mainly innocent from what I known. She changes her picture on there quite often as well, and I find that really weird like she wants to be wanted.

Over the past few days, I saw KIK messages popping up on her phone. And I am thinking that's really strange because I feel that app is synonymous with sexting in my opinion. But at the same time, I thought well maybe shes meeting "friends" on words with friends and then someone suggested moving the conversation over there or something.

She has got so much more protective over her phone. And shes always on the bloody thing. So her phone was just there on the side and I must say I did a really bad thing I went on it ... I know that's awful okay, it is an invasion of trust and it is not like it's my marriage. But over family is so close and I just had to see for myself.
So I clicked on one conversation. And there you have it all pretty much sexting in some way or another, even if it was her talking about what she did with my dad (puke puke puke). Then she sent a picture of herself (not nude or anything). I am just like what the **** mate. WHY.
I didnt have the balls to check the other conversations tbh.

What do I do? Do I confront her? Ignore it - none of my business? Tell my Dad?
I actually feel heart-broken. She is acting like a weirdo. Could it even be menopausal symptoms when she's been acting weird for over a year now.
To me this is cheating, maybe micro-cheating, but still far from okay. I understand in older age you need to feel recognised, but I also think how she's acting is just a slippery slope, she'll probably start hooking up soon so yeah.
CURIOSITY KILLED THE CAT.

How old are you?
Imo its a massive invasion of privacy you going on her phone.
For the most part I dont think its any of your business and you should think carefully what you hope or think you can achieve? Its likely if she was actually cheating, then its a symptom and not a cause of whats going wrong in the marriage.

What I would say is fairly certain is if you tell your dad, then it really will hit the fan.
If you tell your mum, then her trust in you and relationship will never be the same again.

Think before you act.

20 but the thing is that is just our family going on peoples phones. Something my mum has always done. Because you don't have anything to hide.

Anyway I spoke to her about it. She was actually quite glad that I did because she said its a self-esteem thing. She feels no attachment to it, but just feels like it makes her feel better because she's lost herself. There are so many more deep routed issues and she's now going to try out therapy. A lot of horrible things happened to her as a child, and now me and my sister have grown up at uni etc. She feels lost, and doesn't see her purpose.

I am glad I spoke to her about things as what I saw was not what was happening. And she has so many more deep routed issues that might have led to much much worse consequences

20 but the thing is that is just our family going on peoples phones. Something my mum has always done. Because you don't have anything to hide.

Anyway I spoke to her about it. She was actually quite glad that I did because she said its a self-esteem thing. She feels no attachment to it, but just feels like it makes her feel better because she's lost herself. There are so many more deep routed issues and she's now going to try out therapy. A lot of horrible things happened to her as a child, and now me and my sister have grown up at uni etc. She feels lost, and doesn't see her purpose.

I am glad I spoke to her about things as what I saw was not what was happening. And she has so many more deep routed issues that might have led to much much worse consequences