Your problem is almost certainly caused by the stress of exams.
Also; I have noticed that my digestive system is adversely affected by alcohol, so I would suggest giving up alcohol completely until you have passed all your exams, (including SIM checks).
This is worth doing while revising for and undertaking exams anyway - you don't want to introduce poison to your brain cells at this critical time - you need them able to work at maximum efficiency. I would also give up caffeine completely to allow your (deep) sleeping to improve - sleep is very important when learning.
There used to be a medication called Kaolin and Morphine which was very good for settling digestive systems. I don't know if it is still available, but there is now a tablet called Bisodol that I have used, and which might help you too.
Good luck.