What happens if you are given antibiotics for infection and then you find you don't have one and stop taking them?

Nothing: Likely, nothing obvious if this happens a lot, you could develop resistant bacteria, so no antibiotics would work.

Resistance/diarrhea?: Taking antibiotics when not needed can lead to developing resistance. Not finishing a prescribed course of antibiotics can also do the same. It comes down to a matter of percentages. Let's say that antibiotic x can kill 99.99% of bacteria and you have 100, 000 colonies. After you finish your antibiotics, you're still left w/10 colonies that have survived & learned to resist this antibiotic.