There are a few different tests he'll probably want to run--FSH, perhaps being the first. You'll be tested on different days throughout your cycle, but it might be messed up if you're Oing on CD21. Definitely tell him that. Take in all your charts and other fertility records. That way, he can't send you home to chart, etc.

However, I have a sneaking suspicion that he will tell you that breastfeeding is causing your problems right now. The other problem is that many of the meds you could be prescribed to help you may suppress lactation, so your breastfeeding may come to an end, anyway. I'm sure he'll go through the tests if you want them, but the majority of the time, breastfeeding is the culprit.
Do you mind if I ask: did you try for DD, and if so, how long? Your OB may consider this in the testing, want to know about your problems then, etc.
Sorry it's such a difficult road! I hope that your OB is able to give you LOTS of answers and options! Is your appointment specifically for this, or is it a basic check-up?
Good luck, and please keep us updated!