Ink, antibiotics can "maybe" interfere with birth control, although I'm not sure what the effect would be on Depo (more familiar with its effect on the pill etc). Here's a wikipedia link:
http://en.wikipedia.org/wiki/Antibiotic#Contraceptive_pills I would definitely make an appt w/ your GP if you are having unusual bleeding, and, before you go, sit down and make a "normal"/"abnormal" chart so you can detail EXACTLY what has changed. Sometimes it helps to have all of the symptoms clearly laid out (i.e. normally I bleed for 2 days of very light flow; I am currently on day six of very heavy flow...).
As you can see from the link, no one is positive how "much" the antibiotics would interfere, but it's ALWAYS okay to go ask! If you can't get in with your GP, and you have a sexual health clinic nearby, they would be a good bet--odds are they see tons of people in your situation and might be more aware of everything. (the nurses at my local sex health clinic are AMAZING).