I'm afraid I can't be a huge amount of help, but (according to my old doctor, anyway) not all antibiotics affect HBC. It's worth checking the instructions on your antibiotics (if you still have the pack) to see if they mention everything.

If your antibiotics DO affect HBC effectiveness then I would always wait seven days after finishing the antibiotics course to have (otherwise unprotected) sex, just to make sure your protection is back up to full. If you use barrier methods, obviously you can have sex before then, if you're feeling up to it!

If I was you, I'd wait a few days and take another test (making sure you follow all the instructions exactly), but try not to worry - probably just your body getting back into its routine :)