Progesterone is needed to sustain your uterine lining, and to support and sustain a pregnancy. I've been TTCing for 7 cycles and am getting all this checked out by the doctor as I have a short Luteal Phase and I spot before AF arrives - all clear indicators of low progesterone.
The way I see it, is that if my uterine lining starts to shed at 5 DPO (which it did last cycle), it would be impossible for implantation to happen.
Go back to your doctor, let them know your TTC and find out more about what your level of progesterone means to you. Sometimes it means that although you are ovulating, your corpus luteum is not releasing enough progesterone after you ovulate because you are having ovulatory issues - then that is a different story.
I chart my BBT and discovered that my Luteal phase varies from 10 - 12 days, with spotting that starts anywhere from 5 dpo to 10 dpo. BBT charting is a helpful tool with anyone who has LP or ovulatory issues.