Negative turned positive!

Okay ladies. I took a pregnancy test 2 night ago and it read negative, 2 morning later I took a test at about 6:30 AM, test came out negative. I just laid it on the bathroom counter and went on about my day... Went to the bathroom later and just happened to look again and there is a faint line on both tests, but the newest one looks a little bit darker..... Has this happened to anyone before and what was the outcome? Today is 8-31 and my period was supposed to be here on 8-25 but never showed up..... Any advice?