Generally to check if you have a progesterone issue you have a BT on CD21 (or 7 days after ovulation).
So they can test when you are not pregnant?

No not a IVF cycle at the moment - I am having a few months break after the last one to let everything (and my head) settle down again and from what my FS was saying - it will be a natural cycle again as I still have 2 frozen embyros.