You never get a false pos if you got the result within the time frame (usually 5-10mins) & you are not on fertility drugs. So if that is when you got the pos you are preg so congrats. I would suggest going to the doctor to confirm with a blood test. I have just found out I'm preg & I got pos & neg depending on when in the day I tested. ↑
That is true, the tests you buy in the stores range in sensitvity from 25miu's to 150miu's. The negative you got was probably a less sensitive test. I would get a First Response Early Results and test again. Restrict your fluid intake and hold your pee for about 4 hours, then test again. Let us know. ↑