wait to see if your cycle gets heavy like a normal cycle. if it doesn't wait until you're supposed to start your cycle and take another pregnancy test. cause dark blood normally means its old blood.
If your on birth control, your bleeding, & a negative pregnancy test I'm guessing your not pregnant but maybe get off the birth control & try some more
Idk what to tell ya but Implant bleeding isn't dark red. It does sound like your period is coming on. And since your antibiotics cancelled out your birth control it's very possible that your period will be irregular this time. If you are trying to get pregnant then stop the birth control and if you are not then you need to use back up birth control like condoms on days you take antibiotics. That's all te advice I can give ya!
The thing is this has never happened...im on birth control so it controls my period very well. But I had sex while on antibiotics and that completely gets rid of birth control.
If its four days before your period and it's dark blood it sounds like your period is coming on early.
Id wait until u miss ur period then test...to see if u r.best of luck