I also think it is the anti-depressants.
I came off mine recently as they were interfering with other meds, and my cycle changed dramatically.
I am though, on Depo Provera so that could also be why.
If you do not want a baby right now, I would consistently use condoms (at least) if I were you. Withdrawal can be effective, but condoms are more effective, so I'd stick with those.
Obviously if you're next period doesn't come, do take another test. Let us know how you get on
