Trying to Conceive

My husband & I are wanting to start baby number 2 but I'm currently breastfeeding so I'm not having a cycle to tell when I'm ovulating. I've tried researching & we've bought/used ovulation tests but it changes every month. I monitor my  CM but by then it's too late Dr. says we must try to conceive within 24-36 hrs before. Does anyone have any advice on how to tell when your ovulating without expensive tests?