I had a gastric sleeve done Sept. 3, 2015 and now I feel a lump underneath the left side incision. What can that be?

This is probably healing tissue from the recent surgery.  In surgery it is called the "healing ridge".  As far as feeling gas pain after eating, eat slow, do not drink or swallow too fast.  Swallowing too fast can cause gassy pain and disconfort this soon after surgery.  

Hello. This maybe normal if your surgery was recent. If not and you had surgery a while back it may be a sign of a developing hernia. The best thing to do is to get checked by your doctor. If you recently had your sleeve some pain, lump or hardness is expected there. Here's a video that will explain more. I hope this helps. warm regards, Dr. Alvarez