Uh, progesterone alone, I'm not sure. I would not recommend it though, as I've never seen it. But I believe I have read that progesterone weakens bone. If you start taking it alone, not only will you feel really weird (increased libido, appetite, heat flashes, etc.) but I think you will also accelerate the destruction of your bones. There must be a reason why they don't prescribe it alone usually.
And see your physician. "My bones feel weaker" is not very objective : you may be imagining it. And in any case, it is always better to see your physician before making any change to your medication, including hormones.