fibroids the reason?...

I had sex(protected) on Sept.29 My period was due on the first week of Oct. I spotted from the 2nd-4th and it started the 5th. My flow usually lasts 2, maybe 3 weeks due to fibroids. This one kind of seemed shorter...Oct.5th-12th then two more days of spotting. I know it wasn't implantation bleeding, it was too heavy. Since mine happen so often(fibroids) my next one was due on Oct.25th, two days ago but hasn't showed up yet. I took a pregnancy test this A.M....BFP! I'm shocked. I haven't had sex since the mentioned date and had a normal flowing period, yet I got a BFP. How is that even possible??