IUD and TTC first month

I got my IUD out on March 31st. Had one period at the beginning of May. Extremely heavy beyond norm for 4 days and that was it. I've never had regular periods even before birth control. We started ttc on may 16th. How can I figure out when I'm ovulating with buying the tests? We are on a strict budget and only have it budgeted for preggo tests. Has BFN last night and this morning. Sad but know it could be too early. Thoughts?