False positive?

August 3rd my boyfriend and I had sex and later that night he told me he came inside me. It's 2 n a half weeks later And my periods Is late. I took a 5 day sooner pregnancy test and it came out positive but the lines weren't super clear. So then I went to the store and bought 2 regular pregnancy test and they both came out negative. Is it possible that the 1st one I took was a false positive? Or could it be that I am pregnant and I just picked up on the hormones because it's a 5 day sooner test, and that the to regular pregnancy test that I took didn't pick up on the hormone so they came up negative.