Osteoporsis and Losing weight

My mom (75 yrs old) was recently diagnosed w/ osteoporosis.
She is taking Fosomax (1 pill/month). She has lost about 20 lbs in the last two years. She weighs 107 lbs now.

Her main complaint is back spasms. She said her doctors don't seem concerned about that and the spasms started before taking the Fosomax.

Do you think the drastic weight loss could be the reason for the spasms? I've tried to get her to increase her calorie intake and to exercise.
How can we help her to get rid of these spasms?