Muscles need potassium to contract and small changes in the level of potassium that is present outside the cells can have severe effects on the heart, nerves, and muscles.
Have you been ill such as diarrhea and throwing up? Low levels of potassium can cause muscle pain and weakness, have you asked your Dr does he/she think the low levels and your back and hip pain are related? I too agree that you need to get sufficient pain control, if all your taking is the soma and valium then you need to have your pain addressed by your Dr. Also if you do not have a pain management Dr you really should get one as they can offer you many more options than your primary care Dr.
I wish the best of luck and suggest you do get a PM Dr and this will help you with your symptom's and you will receive better pain management and more answers.
I am so sorry you are still in pain. Have you discussed this with your PCP or your surgeon? If you aren't sleeping well due to the pain it will interfere with the body's ability to heal. Additionally when you are tired you hurt more, at least most people do.
Are you taking medication for B/P or a diuretic (water pill)? Both can deplete potassium.
In my opinion you need to get your pain under better control. Again I would recommend you inform your doc's about how much pain you are experiencing.
Please keep us posted. Take care, Tuck