My first thought was maybe ulcers or excess stomach acid.
Although if it turns out that it is the latter please be careful using antacids. Make sure it's high not low (a simple test is to see if after eating and you have the stomach ache if a couple tablespoons of lemon juice or apple cider vinegar in water helps any - or makes it worse - if better then it's LOW stomach acid). My BF's mother used them for years and we've since discovered my BF who is genetically inclined to the same thing really had the opposite of high stomach acid and taking antacids actually hurt you more and more. I wonder to this day if she wasn't getting the nutrition she needed all her life because of her dependence on the antacids and that's what shortenned her life ultimately.
Going to a doctor is important if this continues - but there are some simple natural things you can try in the meantime - my BF relies on probiotics daily, enzymes with a particularly heavy meal and his "ginger chews". Candies that are high in ginger. I have the opposite - I get high stomach acid from two much acidity, spicy foods and wine, but ginger is also a great tonic for me. Even a teaspoon of cooking ginger (ground stuff you use for cookies) in hot water and honey will help.
The heavy hitters for low stomach acid include charcoal tablets and HCL. But these are for temporary use and I recommend doing some research as there are "ways to use them" to help heal.
Feel free to PM me if you want to alternative treatments first but don't know which way to go - high or low stomach acid. We just went through months of this so I might be able to give you some ideas. But if you don't pinpoint it right away I really recommend going to your doctor.
Food allergies though I'm not so familiar with though but is definitely worth researching - I agree with poster above.
If it's so bad it is keeping you from exericising and immediate natural alternatives don't help, though - you should get this checked out.