Sometimes OTC products like alka seltzer or gasx can help a little with IBS symptoms. If you're having trouble eating enough you could try supplementing with things like boost or glucerna shakes temporarily, but dont rely on that long term. Remember if you are concerned about money that it is always much cheaper to treat a minor health problem when it first begins, than a major illness much later. Please do see your doctor if your symptoms continue, or prevent you from eating well.
I am glad to hear you are staying on track otherwise. Thats great! I know its much easier said than done.