Metronidazole (Flagyl) is one of the antibiotics that has been shown to reduce the effectiveness of HBC, but not to the extent of rifampin. I was hoping I'd find a different answer for you when I searched... http://www.drugstore.com/ask/do-antibiotics-interact-with-birth-control-pills/qxa1091
I'm not sure of your timeline. Was the first Friday you mention the 20th of May? So you started your antibiotics on May 16 and took them until the 22, then had unprotected sex on the 24th, followed by 2 more weeks of active pills and then no withdrawal bleed? If that's correct, a test today should be accurate, but anything earlier could have been a false negative.
That said, skipping a withdrawal bleed doesn't signal pregnancy the way missing a period does - if you get pregnant on the pill you can still get a withdrawal bleed, although most people say it's lighter, later, and often browner (old blood). Not having a withdrawal bleed can just mean that your body didn't build much lining anyway, or the hormone drop didn't trigger a withdrawal for some obscure reason. It's pretty common on some brands of HBC and for some women.
All that said, IF the antibiotics interfered with your pills enough for your ovaries to wake up and you then ovulated sometime between the 24th and 29th, the sex on the 24th could have resulted in pregnancy. After the 29th you'd be safe again - on HBC seven days. I wouldn't think the chances are very high that you're pregnant, but it's possible.
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I'm not sure of your timeline. Was the first Friday you mention the 20th of May? So you started your antibiotics on May 16 and took them until the 22, then had unprotected sex on the 24th, followed by 2 more weeks of active pills and then no withdrawal bleed? If that's correct, a test today should be accurate, but anything earlier could have been a false negative.
That said, skipping a withdrawal bleed doesn't signal pregnancy the way missing a period does - if you get pregnant on the pill you can still get a withdrawal bleed, although most people say it's lighter, later, and often browner (old blood). Not having a withdrawal bleed can just mean that your body didn't build much lining anyway, or the hormone drop didn't trigger a withdrawal for some obscure reason. It's pretty common on some brands of HBC and for some women.
All that said, IF the antibiotics interfered with your pills enough for your ovaries to wake up and you then ovulated sometime between the 24th and 29th, the sex on the 24th could have resulted in pregnancy. After the 29th you'd be safe again - on HBC seven days. I wouldn't think the chances are very high that you're pregnant, but it's possible.