I just got my BFP! I'm nervous it's a false positive!?

I was testing for the past few weeks and I got negatives last week, then I scheduled a blood test with my doctor on Tuesday but I got what I thought was my period in the morning, it was a brick red but not even enough for a panty liner, and it lasted until Wednesday morning when it was a brown mucous. I gave up hope, but I took a test this morning and got a positive! I can't get a blood test done until Monday. I took the test and the two lines came up almost right away and got darker after a few seconds. How often do false positive's happen with Answer brand hpt?