Totally TMI but reading this made my insides hurt. How you managed to have sex 3 weeks postpartum is beyond me. Cudos to you. After I had my first child there was absolutely no way that was happening at 3 weeks. But I took quite a while to heal after.
But I agree with Mags, I would wait a month and then test. There is no way of telling when or even if you ovulated.
Since u don't know yet when ur actual period starts u won't know when u missed it. I would give it a month from when u had sex. Just because the bleeding after ur deliver is not really ur normal period. It takes time b4 it's regular again.