As u know ur BMD level is low that's why it's causing u pain so taking calcium ad vitamin medicin will boost the required level of calcium ad vitamin in ur body. But when the level was low ur body started to pain due to weakness of muscle ad bone ad u were doing ur day to day activity regularly in pian. So still after taking medicine ur level has increased but ur muscles are weak to do activities that's why u r having pain in walking ad doing exercise. So plz consult physiotherapist around u to tell u right exercise for u in right way ad also help u in reducing ur pain by electronic modalities. Don't do exercise by ur self without knowing it's harm ad banifits.