I'm really concerned about this - cystitis or uti?

Sounds like a UTI, think bladder infection. Add about a quart of plain cranberry juice to your diet per day while you are on the antibiotic and eat at least a cup of live culture yogurt every day for 10 days after so you don't end up with a yeast imbalance too. It is a bacterial infection and some women just seem to be very susceptible to them. Wash your hands before you go to the bathroom as well as after. and if you are ually active make him wash his hands and his thing before he touches you.