Pregestserone <0.5 but temp rise?

Jenna

This month was my first time taking letrozole (obgyn prescribed because I wasn’t ovulating). Dosage was 2.5 mg. According to my temps, I ovulated on day 15 (had a sustained temp increase of 0.4) however my mid luteal progesterone test came back as <0.5 😕 Has anyone else had a temp increase indicating ovulation but super low progesterone? So confused.