Check out - fertilityfriend.com - there is a lot of good BBT info there.
If it stays high for 18dpo according to that site - test. BBT does stay elevated when you get pregnant.
Good luck. Not sure about low progesterone. If you have low progesterone, know it and are TTC, tell your dr so you can get supplements.
oh also...if you have low progestrone would your BBT still be high?