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.
My period hasn't came i'm late.
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.