When to take a test?

I think that I may be pregnant, and I want to take an at-home pregnancy test, however I’m not sure if it’s too early to take it.

I know that some pregnancy tests are able to predict whether you’re pregnant or not up to a few days before your missed period. The problem is, I don’t know when I’m supposed to get my period.

I’ve been on birth control since January, however recently (and stupidly) I haven’t been using it correctly. I sometimes skip days, and don’t take my pill, and then at the end of the pack, instead of taking the pills that would give you your period, I don’t take those, and take all the pills that I’ve missed that month, and just "skip" my period that way. I recently found out, that when you skip a day, you’re supposed to take that pill, even if it means taking two pills in one day. So, I assume that for the past two months or so that I’ve done that, the pill hasn’t been working correctly. Is that true? Or was it still working, since I never skipped more than one day at a time?

Anyway, I haven’t had my period since the beginning or middle of July. I haven’t had sex for awhile, except a few times almost exactly four weeks ago. If I am pregnant, which I think I may be, that will have been when it happened. Sooo. Is four weeks after "conception" too early to take a pregnancy test? Should I wait for a little while longer, and hope I get my period?