It's a prescription antibiotic. Considering the side-effects (I had pretty much all of them, it sucked, my fiancee had none) it's definitely not something available over the counter.

Don't worry too much about an extra two weeks. Some women go years without knowing they have it and those are the ones you really have to worry about. You can try to tell them you're afraid you have an infection, maybe you can ask to have them call if someone cancels an appointment. Or try other clinics if that's a possibility.