Start testing??

I was on meds for a kidney infection last month during my fertile week and I'm a week late as of today. I've taken 2 tests and both we negative. My fertile week (if I had started on the 9th) would be end of this week. I'm unsure if I'm just late and period will change from beginning of the month to the end or if I'm just gonna skip this month. Really don't know if I should start opks this week or wait too glow says my fertile week is supposed to be. I don't wanna miss this month!! Any help is appreciated ladies!!!!