Think I have an eating disorder but feel like a fake?

This is more like a rant I guess, but it’s still a question that I need advice on. I will try and keep this as short as I can but it’s kinda long lol.
So I started uni in September and before then I never had a problem with my eating. Never felt guilty or anything. Idek when I started to feel guilty about how much or what I was eating, like I’m trying to figure that out still and I guess it’s just creeped up on me/gotten worse over time. I think I lost weight in freshers due to just having poor diet, freshers flu etc, so lost weight without trying. Then a couple of my friends from home said I looked thinner and I guess I loved it. (Also Ive never been overweight, always just been normal).
So yeah, I guess I kinda latched on to this being thinner thing. Over time I’ve made rules for myself, like if I have something carby during the day like bread then i can’t have any carbs like pasta or chips for dinner. I’ll just stick to some meat and veg. Or, if I’m having a normal dinner like chips, fish and veg or whatever then I usually will wait the whole day without eating so I can eat that dinner without having to feel guilty about it. Same thing goes for takeaways like pizza etc.
I’ve had a few instances also when I’ve chewed and spat food out, like one time I went for a meal with my parents and I didn’t finish my meal so they got me to put it in a box to take back to uni with me, so that night I sat in my room just chewing and spitting it all. Done this quite a few times now but I’m trying not to let it become a regular thing (it’s kinda gross tbh). I regularly think about purging but then thought of doing it scares me a bit lol.
Another thing is exercise. I live on the 7th floor of my accommodation so most of the time I will get the lift but lately whenever I go up by myself I take the stairs instead cause I feel I’m burning off more calories.
Like I said before, before uni I never had an issue with food, never thought about it. Now it’s all I think about. I’m always planning on what I’m going to eat the next day based on what I’ve eaten that day or what else I’ll be eating during the week. I try to consume between 500-600 calories a day but sometimes I will go over depending (lately I’ve been smoking a lot of weed with my flat mate who gets the worst munchies and it’s so hard to control my munchies when I see her eating. Also she thinks it’s weird if I tell her I’m not hungry or whatever cause munchies are real af lol). I’ve had to come up with ways to handle this like fasting all day then smoking weed just before dinner, just so I feel less guilty afterwards.
Now onto the main ‘question’; I feel like I’ve developed an ed but I don’t feel ‘validated’ to have one, and here’s why:
My best friend has had an ed for a few years now. She only really told me about it in the summer, saying she’s terrified of gaining weight etc. Then it got worse I think when she started her second year of Uni in September, like she told me a lot of things about her ed which was sad to hear as her best friend. I said how the stuff she was telling me isn’t normal and that she should seek help and that’s what’s she’s done, so of course I’m so happy for her as she’s getting therapy now and has meal plans to help her recover (she was diagnosed with anorexia nervosa which I know is a very serious illness so thank god she got help before it got any worse).
The thing is though, I said all supportive things to her but now I feel like I’ve developed my own ed, and that’s why I feel like such a fake and a fraud. Like I compare myself to her a lot even though I know she’s not well, yet I wish I was that skinny and could control myself around food as much as her (she could go all day then into the next day without eating anything). I’ve also been obsessed with getting a thigh gap and I’m always checking my body in the mirror to see if I can notice any difference in weight loss.
I checked my BMI and it says I’m underweight (heard these thing aren’t accurate though)
I’m 18, female, 4’11 and have a bmi of 16.4 which according to the bmi calculator is underweight.
I’ve noticed my periods are quite irregular now as well. They’ve been coming around ever 6/7 weeks which is kinda stressing me out (been getting them since 13, always been normal but now they’re not) but I’m scared to go to the doctor about it.
Anyone else ever like a fake with their ed? Do I even have one? I feel like I do but I hate admitting it if that makes sense. Sorry for the length of this for anyone who got this far lol. I’m too afraid to open up to anyone about this (especially best friend as she has her own ed) so it feels good just writing it all here.
Also, having explained about my friend, do you think it’s possible that other people can influence you? A part of me thinks I wouldn’t have an issue with eating if my friend didn’t have an ed. Probably sounds stupid I know.
I’m sick of feeling guilty about eating and always having to plan ahead every time I eat something😒 it’s honestly taking over my life.