When did you start progesterone in your cycle?

Hello! I took progesterone for the first time last cycle and I started on day 21 after having the progesterone test show that my levels were a little low. The nurse I talked to yesterday said to start taking it this cycle after I get a positive opk but what I’ve read online is that that could stop you from ovulating. I have a call into the doctor today but I’m just curious what your doctors have told you about when to start progesterone in your cycle? Tyia!