I completely understand and empathize with you!!! I took prometrium also for a while during the post-ovulatory phase of my cycle. I had horrible mood swings and water retention. I called my docs office and they had me come in to look over my charts and discuss it with my doctor. He believed that it was because of the fact that I was taking it orally and so the progesterone was either high in my system or already out and so I would have a low which would cause the moodiness. Honestly, I felt crazy!!!
About a year ago he switched me to a natural progesterone cream. I use 50mg twice daily during the post ovulation phase. It has helped me a great deal and my husband is so greatful for it!!! It's a compound progesterone that I get at Kubat's pharmacy. I don't get any of the side effects that I got with the prometrium. I feel much calmer too.
I suspect that if you're starting your period before day 25 of your cycle then you're likely ovulating eariler than the typical day 14. Which in your case would mean that you need to start the progesterone much sooner than day 16. Do you chart your cycles? Do you have any idea when you might be ovulating? If you are ovulating on day 14 and you start your period by day 23 to day 25 then you would be infertile. Your Lutuel phase could be insufficient which can prevent a successful implantation of a fertilized egg.
The reason your doctor is having you start the progesterone on day 16 is because he's baseing it on a "typical" 28 day cycle in which the woman ovulates around day 14.
You need to do 2 things. You need to talk to your doctor about switching the type and route of progesterone you're taking and you need to start charting your cycles to know when you're ovulating. Unfortuneatly most physicians don't know or trust natural family planning, which so many women have learned to use and become pregnant with. By learning your fertility cycles you will learn so much about your own body which will empower you!
I know this is a lot to digest. I'm a registered nurse AND I taught natural family planning for 8 years. I taught the sympto-thermal method, which is charting mucus and temperatures. I've since learned the Creighton method, which is mucus only and I like this method now for me. I think both methods are equally wonderful. Just telling you my own experiences.
If you're interested I can give you some great resources for learning more about charting.