What Causes Chronic Diarrhoea With Weight Gain?

My granddaughter has been sick for quiet awhile now...her stomach is swollen and she has chronic diarrhea(she goes about 20 times a day..sometimes more) on most days....there might be 2 or 3 days a month that she doesn t go at all...she is always sick at her stomach...she belches up what she calls nasty stuff and she is weak and tired all the time...the problem is when she got sick she started gaining weight and gained about 50 lbs...and now even with the chronic diarrhea she isn t losing any weight..it just stays the same...what cold be her problem?