Hello !
If you have upper digestive tract problems like stomach aches and nausea, I would definitely suggest ginger tea or fresh ginger for nausea. If you have stomach cramps, I would suggest getting OTC meds which protect the stomach lining when you eat and from other meds.
For diarrhea, I suggest you eat as cleanly as possible (enough fibre so you don't get constipated, but I also suggest rice so you don't "empty" yourself all day)
Imodium (Ioperamide) helps again diarrhea really well. Again, there are some OTC meds against the spasms your intestine gets. This would help with the pain associated with diarrhea, if you happen to have pain.
I do think these tummy problems come from the food change and from eating a lot of junk food when you were at college. However, if it doesn't subside within a week or two OR if you get a fever, you should see a GP asap. Hope you'll feel better soon !