Pregnant?

I am currently 15 days late on my period. Last weekend when I was 10 days late I took some cheap pregnancy tests from the dollar store and I believe I saw a faint line on one of the tests. Now I am 15 days late and waiting to take another test. I started having cramping last night and I have a very tiny amount of dark dark discharge when I wipe definitely not my period in amount or colour? Could this be implantation bleeding or is it likely that I am just beginning my period? Should I test again or wait a few more days?