About 1 in 5 people who take antibiotics develop antibiotic-associated diarrhea.
I've had 'random' back pain on and off for a few weeks now, the amount of people who get random aches and pains may surprise you, if you haven't heard about it its because its usually nothing, and so common!

If you're not satisfied with your GP's response then I highly doubt any of our responses will satisfy you either...
have you spoken to your GP about your health anxiety?