Ttc and ovulation

Kendall
So I tracked my ovulation for a while at the beginning of ttc then gave up because it got super stressful and depressing. I took a test on the 9th and it was positive then took a test on the 10th and it was extremely positive and we had sex that evening. Well I tested this evening and it was a double line but not both dark which means it was negative my question is can your levels change that fast?! I am nervous because we weren't able to have sex yesterday until 9pm and when I tested and got the positive it was around 12 in the after noon. I'm wondering if we missed our chance /: two years of trying and I am trying for two more months before doctors will help me.