I feel gross after I eat?

So, lately I’ve been feeling really gross after I eat. I get headaches, I feel nauseous, I get really lethargic, etc. I’ve been a vegetarian for about 7 years now and have never had an issue with this before but I have no idea what else could be causing it. I feel like the food I eat is just sitting at the bottom of my stomach if that makes sense. And I mean, I try to eat healthy/exercise as often as I can.

Does anyone have any suggestions for what I should do? Or if there are any vegetarians/vegans out there, what vitamins do you take if any?

Thanks!