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First thing you need to understand is that due to recent pregnency all the muscles becomes weak. specially of lower limb and back. so you need to giver her appropriate rest as stress
or exercises can exeggarate the conditions. Just go for hot fomentations to back and knee. after getting some relief from pain just start spinal extension exercises for back muscle strengthening and isometric exercises for knee by putting a roll of towel under knee and pressing the knee and stretching the toe in the direction of your head.
All the Best and wishes for quick recovery
Dr. Udyan Bhatnagar