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.
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.