The tests that measure the highest amount of hcg- as in 150- are most reliable in terms of "Yes you really DO have lots of pregnancy hormone in you!". However, most people like to use ones that say they measure 25-50, and in fact, that's what the "early test" kits do- 20-25. The problem with those is that if you leave them sit past the time frame, they CAN turn positive when you're not really pregnant. That, and/or you might have another issue going on that was picked up by the test. (Just happened to me.)

All in all, buy another test and retest. When I've been pregnant "for real", the tests showed positive at the time my period was due.