How do you know u're conceiving after had sex?

Hi guys! I have a rare condition (I got my period once in 3-4 months), so I literally don't know when my ovulation begins. So I just had sex like 3 weeks ago and I just wondering how do we know if we're conceiving after that sex (besides using test pack). Do we feel cramp, dizziness, back pain or something? Please tell me!! I'm hoping so much I can get pregnant soon 😀😀