Calendar says I'm ovulating but no LH surge?

My husband and I have been TTC for about a year now, and I've just gotten the go ahead to TTC after a 8 month bout of very aggressive carcinoma in situ. Kinda freaking out that now just when I've been cleared I'm finding something else is wrong with me. Has this happened with anyone else? I'm using the first response ovulation kits, and they seemed fine, but no LH surge on what's supposed to be my peak days?