Line not getting darker?

I'm 4 weeks 2 days and have been cramping on and off basically since ovulation (2/21). Spotting yesterday, for like a second. This morning I started spotting, more cramps. Came home and tested again and the line is exactly the same as it was 3 days ago when I missed a/f. Shouldn't the second line be a bit darker by now? Has this happened to anyone?