What Causes Recurring Abdominal Pain Radiating To The Back Post Antibiotics Intake?

I had been prescribed an antibiotic and had been taking it for a week for cellulitis. I noticed that after my evening meal the other night that my stomach got full fast (haven t had much of an appetite anyway. side effect of the drug) and started have pains in my stomach and radiates to my back. It went away after about 1/2 to an hour. I the next morning took my next dose of antibiotic (prescribed 1 twice a day) That night about the same but I did not take the evening dose. Thought I was having side effects. Also noticed my tongue to be sore and sensitive to even bread felt like rough sandpaper on it. The next evening (last night) I ate my meal and before I even finished the pain started happening. Pain was high under my ribs more on left side and radiated throughout my whole middle stomach area and felt like it was coming out my back. It was so painful. I would say equal to giving birth. At one point I thought I was going to vomit due to the extreme pain. Then it began easing until it was gone. The whole entire episode lasted somewhere between 30 to 40 minutes. Could this have been a side effect of the antibiotics or something more serious? Today I have had liquids, an apple and some chicken and rice soup. Only have a dull pain (tolerable) again, only when eating solids. Antibiotic is Sulfamethoxazole-TMP or Bactrim DS