It might be a vitamin deficiency causing the cramps, or sciatic nerve pain. you can take a multivitamin (if you don't) and eat things high in potassium (I am allergic to bananas too, I thought I might be the only person allergic to them) and see if it helps. Also can get your levels checked at the Dr.
For a rice bag, if you sew, you can get a travel pillow case, cut in 1/2 (for a small size, or if you want a really big one don't cut it), turn inside out and sew where you cut, fill 1/2-2/3 with rice (you want it to still be flexible, not stiff), and sew the end closed, use fray check or a zig zag stitch on the rough end(s) of the fabric. Make sure you buy a cotton pillowcase, polyester and other synthetics might melt in the microwave.