make sure you were checked for any kind of post-birth infection. while infections arent common, they do happen. only other idea is you messed up your back and are having some kind of pinched nerve issue. go to a chiropractor if you think this could be the cause. a pinched nerve will cause 'strange' and out of the ordinary things to happen to your body, that will easily come and go, depending on the amount of pressure to the affected area. simply turning your head an inch can cause a pressure difference.
Hello I agree there could be some hormonal issues going on. You could have an infection of some sort that your body is having trouble fighting. You could be suffering from exhaustion. How long ago did you have your baby? Do You sleep well? Are you eating a good diet? The neck and upper back could be a pinched nerve in your neck or a disc issue with your spine just to name a couple of possibilities. The weakness could be from an infection or problems with your levels such as thyroid. It is really difficult to say the exact causes of your symptoms but I would recommend another trip to the doctor for labs/x-rays to try to find the answers. I Hope This Helps. Good Luck.
Chills and headache sound hormonalBack pain probably is postural. If MD didn't do hormonal testing, ask for it.