Hi Wattle,
I have jumped in without reading the other responses as I have to go.
Do you think your doctor meant a 7dpo progesterone level? This is also often termed 21 day progesterone level. However the 21 days is misleading as to test on cd21 would be inaccurate in a woman who has a 38 day cycle - iykwim...
Anyway, usually this test is done to verify weather ovulation has taken place. It is done 7 days after you ovulate. It is just a simple blood test.
It is usually the first port of call just to check out if your body is doing its thing correctly.

Lots of love,
Deb