Nancy, sorry to hear that you already had 3 stress fractures. Since, you have arthritis in the neck, and herniated discs, be very careful with the exercise you choose. I've been on prednisone for several years for a chronic disease, and developed cortisone induced osteoporosis. Now, I'm on a drug called Actonel for strengthening and improving my bone density. I was also told by the doctor to take calcium and walk a lot. I also lift weights (at times) for the purpose of building bone density, although I haven't done it for awhile. I usually start at 3 lbs and after a while work up to 10-15 lbs (in each arm). My last bone density test showed that I lost a half an inch. I'm only 5 ft and a 1/4 inch tall, so hopefully I won't end up to be 3 feet tall, that's why I have to make a good effort to do my exercises.
The swingey, thingey things under the arms is the tricep muscle that decides to pack up and go on vacation. When I notice my upper arms getting loose, I do the bicep curl, but for the underarm I do the tricep kickback. It's a simple exercise for the underarm and results come quickly (atleast for me).
Good luck.