Letrozole Fet Cycle

[SAEMBY]

Hi I am looking for a little advice. I have had 2 rounds of ivf this far. Our first round we only produced 1 embyro that did not make it to blast to freeze, our 2nd ivf using different meds we achieved 9 eggs 5 of which fertilised and 3 making it to day 6 freeze -BB grade. I just recently did an FET cycle which was unsuccessful using progynova. I found the progynova dreadful I felt it switched off my body and caused very bad mental side effects. I saw my specialist yesterday in prep for another FET. He has switched my meds to Letrozole crinone and oripro. My question is during my stim cycles I was unable to do a fresh transfer due to progesterone rising too soon. I queried this FET protocol being more natural whether the same can happen my specialist advised me that it is only a problem with a fresh transfer and will not affect a frozen transfer.... why is this different? I would hate to have to cancel cycle due to high progesterome. Any advice would be great!! ❤