Any of you had false positive dollar tree OPK?

One of the drs at the fertility clinic recommend I use dollar tree opk, I was like, ok, I had clear positives on CD14-15 and then another on cd 19, since I'm taking letrozole I had to get a progesterone blood test yesterday to see if I has ovulated which I was almost sure because of the tests, well, turns out they where wrong and I didn't ovulate... I told the nurse about the positive opk and she said they had he same problem with other patient and according to her, the dollar tests pick up small amounts of the hormone and it can give you a false positive, I'm still a bit confused about that but I guess I'll just wait it out a couple more days and then onto the next cycle, any of you girls with similar experiences?