DNA testing

I posted a post a few months ago, here's a little reminder
I had sex with A on June 10th and a few days on and then B June 20th. I tested positive the 26th. 
I asked everyone who the babies daddy would be, everyone said definitely person A. 
Well I'm O- and my daughter is O+ 
Person A is A-  
Person B doesn't know what blood type he is. But his mom is O+ so that means there is a change he is also O+   
Well, I asked the NICU Doctor and he said there is no way that two NEGATIVE parents could make a + baby. So that rules person A out. 
So i guess my question is, how did I get pregnant so fast... In just 6 days.. 
I didn't think it was possible.