Letrozole no ovulation

Nik

Second time ttc with letrozole . I’m cd 16 and tests sooo negative I’m concerned it didn’t work :( when I got prego with my son on letrozole I already had positive opks starting cd 16 and ovulated cd18 . I did have extremely bad back and leg cramps last night but test still very negative this morning , anyone had a similar experience ? Here is my lh and bbt chart