Can only get semi- for last 3 weeks...hate it...why?

I'm only 17 and had no problem at all having before but then she went on antibiotics so the pill would have been ineffective and I had to use condoms. Since that day I have only been able to get a semi-, not a rock hard one. Even though I stopped using them about a week and a half ago, it still hasn't gotten any better and it's really worrying me. I've read about psychological issues but I think I'm fine in the head so I don't know what's up!!