Whose baby is it? Young and dumb and need help!

In February 11-21 I had sex with my on and off ex boyfriend. Condom 
In February 22-25 I had sex with someone else. No condom
But on February 6 I got a negative pregnancy test.
Today is March 8 and I'm now pregnant! The doctor says I could be 11weeks or way earlier than that but I'm thinking 11weeks isn't correct if I got a negative pregnancy test on feb 6. The doctor even says that she doesn't see anything on the ultrasound but she calculated it from my last period which was in December and by the wheel thingy they use to determine how far along.