I had this happen to my littlest one about 2 months ago. He's only 3 lbs and I swear I could see his belly getting bigger and bigger.
I called the Vet and after confirming that he is moving his bowels and no other symptoms are happening, she pretty much told me he has gas.
He was allowed 1/4 of a Tums tablet. I got him to eat it, and his little belly went down in a flash.
