Porgesterone blood test

Just wanted to ask last week doctor told me to have my 21 day blood test, today i went to doctors to get result and she was telling me that i should have done it 7days before my period & the results are showing low?? I have a 33day cycle i dnt know why the other doctor sent me on day 21 surely it would affect the results wudnt it...i still have to have another blood test for when im on my period & doctor have requested me to have a scan aswell, has anyone else had these tests and how do they work?? Im so worried