I know that progesterone medication doesn't always show up in blood tests. However I've always taken progesterone in suppository form so I don't know if it's the same for the injections.
My clinic doesn't even bother testing progesterone levels in early pregnancy if you're already on supplemental progesterone, I guess they just assume that the dose they've given is enough to sustain a pregnancy!
A progesterone level that is insufficient to sustain a pregnancy will usually manifest via spotting, so if you don't have any spotting that's a good sign!
The danger with taking your extra suppositories is that if you don't have enough to make it to 10-12 weeks and your Dr doesn't prescribe you more, then you'll have a sudden drop in progesterone level when you stop taking them, and your body might interpret a sudden drop in progesterone as a sign to shed your endometrial lining which could be dangerous for your pregnancy.
Me (32), married to DH (36)
3 years of TTC #1, M/C @ 6wks in May 2013
Mom to Liliana born May 2014