antibiotics and birth control pills

  On Sunday afternoon, my husband and I had sex.  I am currently using Ortho-Cyclen 28 and took my pill at 7:00 p.m. as I always do.  Then yesterday, Monday, I went to the OB-GYN and they said I had cervisitis and gave me a 1 gram dosage of Zyothomeria (?) that would get rid of the infection in one dose. I took the one dosage (4 pills = 1 gram, I think) at around 6:30 p.m. and then took my pill at 7:00 p.m.  We didn't have sex on Monday when I took the antibiotics only on Sunday afternoon.
  I know that I will now have to use another form of birth control this week if my husband and I decide to have sex but my question is this:  Will that dosage of antibiotics taken on Monday night affect the pill from the day before, which was Sunday when we did have sex?
  I assume that antibiotics only affect the pill the days they are taken and thereafter and that is why patients are told to use another form of birth control when having sex on the days when antibiotics are used and days thereafter.  I just wanted to make sure the antibiotics won't affect the pill from the day before when we had sex.
  And how long should we use condoms now that I have taken the one dosage of this medicine?
  Thanks a bunch for your help.

[hfhs.rn.bp]

Antibiotics do not affect anything that has happened before they are in your system.. Very rarely do they affect birth control pills.  If you experience any spotting, this may be an indication of decreased effect of the pill and y ou would need to use condoms.  If you want to be safe, 5-7 days of condoms could be used.