The easy simple answer here would be gas. Try over the counter beabo or gas-x. Also, you could be dealing with lactose intolerance
, try a week with no dairy products and monitor for improvement.
To answer your question, "is it anything serious" is very difficult. The most worrisome thing would be a tumor (benign or cancerous) but that is not very likely. However, in order to get to the bottom of this you need a medical work up.
Best of luck.