Some reasons for a false positive:
-cyst
-if you're taking any medications that contain HCG
-if you've recently miscarried or given birth
If your test is negative and you're late, there are many reasons why your period could be late.
It could be a false positive because your on fertility drugs or it be negative because you aren't far enough along to get a positive. However its rare for them to be wrong all the time. Usually if it comes up positive you are negative your not.