Welcome to HW!
Your colorful stool is most likely due to the antibiotics - especially if it just started happening when you began Penicillin. IBS is mainly a change in bowel habits and pain in the abdomen accompanied by gas.
Antibiotics kill off the good bacteria along with the bad. You might want to get on a probiotic while you're taking the antibiotic. Either that or eat lots of yogurt for up to a week after you stop the antibiotic.