@VanGoSunflowers
In terms of skipping lunch and stress etc, I would advise making a conscious decision to eat every meal and perhaps try some meditation and breathing exercises (if you haven't already) to put you into a calmer state). I feel weak and weight falls off me fast if I skip meals, so it's easy to put it on (if you want to) by eating every meal and allowing for treats (if you want to).
I've no idea if you're too skinny based on measurements and BMI as it all depends on a person and how they look and feel, their bone structure, fat composition etc. I look much thinner in clothes than naked. I have so much fat on my body that is well hidden by clothes so people often think I'm thinner than I actually am!
As for your colleague, I would take being told by someone that I'm 'too skinny' as an unwelcome comment. I learned a few years ago not to comment on people's weight. When I was younger, I used to comment on weight loss etc. as most people I knew did and I thought it was the norm to compliment and comment. Now I simply say nothing and if I ever say something, it would be to say, 'you are looking really well.' One friend of mine who had been obese and had a host of health issues as a result lost a lot of weight; so I told her she looked 'absolutely wonderful' but I never commented on her weight.
Another friend lost a significant amount of weight - I later found out it was anxiety-induced - and thankfully I did not comment on it. Yes, she looked unwell at the time because she had been unwell mentally which made it difficult for her to eat. She has now put on weight, looks way better but again, I did not comment.
I'm not sure any of us like our weight being commented on, do we? We like comments such as 'you're looking great' etc. but I don't think we like the specifics being mentioned and no-one wants to be told they're 'too' anything.
So, my point is that your colleague's comment is hurtful and I would be hurt by it. I know it can be difficult to be direct but if she says it again, could you try saying something like, 'do you mind not saying that again? I'm sure you don't mean to hurt me but it actually really hurts my feelings when you say that.' If you'd find it too hard to be that direct, the next time she says 'you're too skinny,' you could say, 'oh let's not talk about my weight again!' or 'you're always commenting on my weight.' Either way, I think it's important you say something in a way befitting to your personality to get the message across that you do no want your weight to be commented on again.
Best of luck OP. Wishing you health and happiness.