Hi i have a pain on my upper back (core) and buttocks when i hop / skipping the rope. and my lower back pains when i stretch / touch my toes... any ideas what could be wrong?

Somatic dysfunction: unless you have a history of major trauma to your skeletal system, surgeries, plates, screws etc...and no chronic spinal conditions, no symptoms of nerve compression such as numbness and tingling; then I would try a chiropractor or physical therapy. If that does not help I would see a Primary Care Provider for further evaluation.