Okay....... I'm probably going to get skewered here but.... I have a different opinion.
Whereas ones sexual interactions are (or should be) kept private I feel you can and should discuss some of this with your mum. As a previous poster wrote he/she was still playing Club Penguin when they were your age. You are young. That is a fact. The problem with starting such intimacy so young is that you are more likely to enter the next stages of sexuality sooner than is probably a good idea. You may be young but you and your bf are old enough to have been interested and liked touching each other. Then what happens next......... assuming you stay together as a couple there will be more touching and even more intimate touching which can and most likely will lead to different sexual activity potentially leading to full on intercourse. If one is of age then it is just between the two consenting partners but that is not the case for you. Do I think you should come out and tell your mum everything you did with your bf - probably not - at least not yet. But if you are going to start down the road to sexuality and you are blessed enough to have a relationship with your mum that you can and do tell her important aspects of your life then I feel you should start talking with her about sex in general. The problem is a whole lot of really nice but young girls end up pregnant as teenagers. Which conversation do you feel your mum would rather have with you..... "Mum, I want to be honest, "BF name" and I have kind of been doing a little more than kissing, I let him touch me and I touched him" (and see/hear her response) or would she (or you) prefer "Mum, I didn't want to tell you because, ew, talking w/ my mum about sex would be weird so I didn't but "BF name" and I have been fooling around and well, I've missed my period for a month now and I think I'm pregnant" or perhaps the conversation would be "Mum, "BF name" and I have been fooling around and I really didn't think it was something you'd want to hear about but anyway it turns out I'm pregnant. I'm not sure whether I should just get an abortion but "BF name' really doesn't want me to so could you help us raise this baby? I realize my whole life is going to change and be a whole lot more difficult".
Honestly, most people just don't have parents they feel they can talk to but if you trust your mum and don't think she's the type to utterly freak out maybe now is the time to start having some real discussions and not wait until you're 2 1/2 months pregnant with a lot of really overwhelming decisions to make. You don't have to tell her you did anything but I think you should start a conversation about sexual experiences because she is someone you feel you can trust. Also, I'm guessing she also feels close to you and trusts you as well don't risk her trust in you.
So, you haven't done anything wrong but this can go quickly to a place you may not be ready for...... be cautious......