Stomach pain

Following a 10-day cycle of clindamycin (which caused significant acid reflux at the time), prescribed for an abcessed tooth, I suddenly began experiencing mild stomach discomfort upon swallowing food. It seemed to occur as the food entered my stomach and also seemed to be ocurring under the breastbone and near the lower portion of the left rib cage. Now, three days later, the stomach discomfort has increased to moderate with an occasional mild burning sensation between the navel and breastbone. It still seems to be triggered by eating but it also hurts from time to time in the absence of eating. Is this indicative of gastritis? A peptic ulcer? GERD? Or something else entirely?

I should add that although I've experienced bouts of it, I have no history of chronic acid reflux and no history of stomach problems.