most likely a Neurological problem. do not despair though it could be something that will heal spontaneously.
can you give more information?
what type of back trouble did you have in the past? have you ever had MRI brain/spine? any recent blood tests?
did you notice any other symptoms or anything unusual in the past months? anything even apparently unrelated like urinary tract infections, bladder symptoms? headache? blurred vision, loss of vision or visual field?
anything going on with your right leg too?
See a doctor right away. That could be serious.
Definitely go see a doctor. It is most likely a neurological issue, such as an impaired nerve, or possibly even an MS attack.