Fading OPK after missed period?

I feel as if I've out googled myself with this question, so I turn to you ladies! I have completely missed my period this month, technically today would be my last day. I have regular 28 day cycles, with a day grace period. Instead of my period, I got an extreme cold and slight pinching cramps on the left side that move towards my back, all around the hip area. No cramping in the uterine area at all. I have taken two different tests, one was a Rexall One Step, the other a cheapie dollar test. Negative with FMU and with a 4 hour hold. I decided to use an OPK on Sunday. The line was very visible, but not positive. Today I did another OPK and the line is barely there,it just looks like a water mark. Extremely faint. If I am pregnant and just not producing enough HCG, would an OPK begin to fade? I know I'm grabbing at straws here, but I'm very stressed about the whole situation. Would an ectopic pregnancy caused that to happen? I've over googled myself the past few days and I'm just desperate for an actual answer. Don't worry, I plan on making on appointment Friday if I dont get a positive HPT or my AF. Thanks for any and all input ladies!