What Causes Intermittent Sciatica?

I had an injury to my back at work and caused bulging in my L4-L5 Si - 2 .... .I have been on medications for three years and also had injections. Why does the sciatica come in one leg then the other or both at times? My medications are naproxen, cyclobenzaprine, fast release morphine and long lasting morphine and have to take zipolocone to sleep at night because the pain is terrible. I also have a burning sensation the comes in my spine, and sometimes feels as thought it is bubbling inside....HELP Thank you