Confused?

I have missed my period for 8 days now, which is weird. I had sex with my ex about 3 weeks ago before he left and he came in me. We were trying for months to get pregnant but nothing was working. I took a pregnancy test 5 days ago, it was a faint positive, but when I went to a small clinic, they said their test was negative. I'm just fairly confused about all of it. I haven't ever missed my period and I have no idea what's going on. People keep telling me to take another test, but I know its just going to say negative.