Well not all positives stick around. If it shows up in the proper time frame and is a true positive---has colour---you can assume you're pregnant. This "period" you had. Was it also a true period? By definition you don't have a period while pregnant, and just bleeding doesn't make it a period. What was the flow, how long did it last, how long do your normal cycles last can also help.
My period hasn't came i'm late.
There's only a few ways you would get a false positive. If you didn't check within the time frame and are seeing an evap. line, you had a chemical pregnancy or you're taking meds that contain the HCG hormone in it. HPT's give false negatives but hardly ever false positives.