In my experience, it means I have to get on progesterone supplements right away, but I am not a normal woman when it comes to pregnancy (or many other things, as a matter of fact). In your situation, I would suggest first not freaking out since your levels are within a safe range, but I most definately suggest staying on top of your levels by going back to your np for a follow up blood draw within the next 2-3 days to ensure your progesterone levels are rising and are staying within a safe range. Like if they were done 2-3 days ago, I personally would go back tomorrow or straight to a draw station and have "Stat" orders on that draw so you could get results before end of business Friday.
Ovulating late in a cycle (day 23 out of a 30 day cycle) can means a luteal phase deficiency, which means the corpus luteum (the place where the ovulated egg came from) has not had enough time to produce a subsequent amount of progesterone to support a pregnancy. It's different if you ovulated day 23 out of a 45 day cycle or something. I would think that would more closely resemble an average cycle and produce a healthy progesterone level, but I'm not totally sure. How long was your cycle? Bottom line: You should not delay in pursuing information or following your np's suggestions.
Personally, I've done vaginal suppositories twice with success, and oral supplements alone once with no success, though oral supplements are very successful for many women. (Again, I'm not normal.) Once I did a combo of oral and vaginal supplements, but I think the vaginal alone would have been enough. My doctor stood by the oral like he invented them or something, and couldn't accept that them not working was the reason my levels continued to drop, though I was on the highest possible dose. I was insistent he give me a prescription for the vaginal which I used with my daughter Riley-Grace. He did, my levels rose, and that's all that matters.
I've read some other posts on here of women doing under-the-toungue supplements, but I think you have to do four tablets four times a day that way, and I'm far too forgetful. The oral supplements (Prometrium) are available at your local pharmacy by prescription. The vaginal supplements are available at compounding pharmacies, and while they're typically not covered by insurance, they seem to deliver progesterone directly to the area where they need to be. That's my preference, and what's been most successful for me. Other women have success stories with other options. You could do a search and probably find loads of information.
Some people I think have luck with topical creams, but I think you have to build up levels for a while. When levels are related to pregnancy, I personally don't mess around. Good luck, mommy!