Thanx for all the info everybody, and 'specially for that link to drugs.com icksters. I went and checked out there on the drug interactions
page for the birth control and antibiotic. From the sound of it, an interaction is pretty rare, but they haven't done a solid study with this antibiotic to prove or disprove it either way.
I did come inside her, but we were using a condom with spermicide. I realize that's not near 100% effective when it comes to killing off sperm but I'm sure it helped. My gf only has about a week and a half until she starts her period, so I'll know whether to get that pregnancy test before too much longer here.