Negative test and still pregnant?

Has anyone had a positive test followed by a negative test a few days later and still have been pregnant? I took a test last Tuesday aND it was a faint positive with in 2-5 mins then on Sat I took another test and it was negative. My peroid was due on Sunday and it still hasn't come and I have no signs of my peroid even showing up. But I have pregnancy signs(needing to pee more, discharge, always exhausted, nausea, headaches, backaches and weird vivid dreams, minor cramps a few days before af was due). Is it possible that the negative tests were wrong? The positive was a right aid brand and the negative was a dollar store brand.