Lines getting darker?

My first month using these opk tests, I started testing after my period but they were all negative and there was only 1 line. So I missed like 2 days and then tested yesterday and got a faint line, and tested again today and it's still faint. Does it look like the line is getting darker. Currently on cycle day 13. Need help! I can't tell if it's getting darker or not...and when is the next time I should test? Tonight or tomorrow? We BD like 4-5 Times on Monday...nothing yesterday, planning on it tonight.