Sorry, I've NRTFT but just wanted to say you're not alone. I have a MIL, who I genuinely love, but her insistence on forcing food on me has caused me a great deal of stress over the years, even once to the point of tears (in private, in the loo!) where I ended up avoiding visiting her for a good few weeks afterwards.
I'm a small petite person, genetically. Mother and father both small people with small appetites. I am the same. I can't eat a lot. It makes me feel very ill. I don't do breakfast, rarely bother with lunch, small snacks and one modest meal at dinner time is all I need. I rarely do desserts. I also have awful IBS. Over-filling has me doubled over with spasms.
MIL knows all this.
At times she has left everyone alone on the food-harassing EXCEPT ME. On one occasion she even got it into a stomping mood because I refused to eat a 5 peoples indian takeaway leftovers from the night before AS WELL AS a totally fresh full dinner of quiche, chips and salad she'd just cooked that evening. Nobody else was expected to eat all that... just me. She insisted I had said I wanted to eat all that. I would NEVER have said that.
I've no idea if she behaves like this with me because it's a jealousy thing, as she's always been overweight and I've always been slim (not a brag btw, I still have a mum tum, cellulite and stretch marks like anyone else, I'm just small with it) or if she see's my refusals to eat until I'm sick as an insult towards her?
Either way, she has a bee in her bonnet about my lack of food consumption and its made me really dislike her at times. It almost feels abusive.
YANBU
Its polite to offer, but it's not nice to continue pressuring someone into eating when they don't want to.