How do i know when i ovulated ?

i am not shure of when i ovulated because i got pregnant some time in may my last cycle was april 29 and my dr goes by when i last ovulated not by my last cycle. so how do i know how far along iam ? i thought i was 7 weeks but i was not i did my first ultra sound and all thay saw was a tiny sac and a bunch stuff moving around and good blood flow my dr says when my hcg levels get up to 22,000 then we can see a heartbeat as of now my hcg levels are at 7,429 so what do you think ?