I've been taking Ortho Tri-cyclen Lo as a form of birth control for about a year (I never miss a day though the time I take it usually varies by a few hours). Each time my boyfriend and I have intercourse we always use a condom. However, we do have outercourse more frequently where we don't use a condom, we try to be careful that the sperm doesn't come in contact with anything, but sometimes we slip up. I know it's unlikely, but how common is it to get pregnant from sperm "swimming from the outside"? A few weeks ago I was on an antibiotic which apparently decreases the effectiveness of my birth control. During that time we had a timing issue and some of his sperm came into contact with my vulva.
I recently had a very short withdrawal bleed, only about 2 days, on about the 3rd day of my placebo pills. A day later this was followed by a thick brown discharge and some cramping and nausea. I'm prone to pregnancy anxiety, but should I be legitimately worried?

"swimming from the outside" is very unlikely. However, I was on the same pills, took a week-long antibiotic and became pregnant a few years ago (ended in miscarriage). It's more likely that the antibiotic increased your pregnancy odds, but of course that's not a guarantee that they actually negated the effectiveness of your BCP.

Based on your discharge & cramping, your period could be coming but without taking a test, no one will know for sure.

Anytime you engage in sex, there is a pregnancy risk, no matter how slight. It might be worth your while to test if only to ease your anxiety.