Legs Tremble/Spasm Upon Standing and on Stairs

Ten weeks ago, while in a cast following ankle surgery, I stood up after sitting for a few hours.  Immediately, my legs began a violent tremors/spasms that lasted for 10 seconds.  I had to hold on to the desk to keep from falling.   These symptoms continue until now, although not every time I stand up.  When the cast came off, I tried to wallk down the stairs and again, my legs tremored/spasmed so badly that I needed to hold onto the railing.  

My first reaction was that my legs were weak from six weeks of not exercising.  The symptoms continued for three weeks and stopped.  I thought again that it was simply weakness from the surgery.  However, two weeks later, the same symptoms reappeared - even worse.  It feels as if my legs are about to buckle when standing and when going down stairs. I have no problems going up stairs. I have been doing exercises in physical therapy to strenthen the legs and my symptoms seem to be getting worse.  When I bend down to pick something off the floor, the legs tremble so bad that I nearly fall on the floor.  

I have terrible exceessive daytime sleepiness that 150mg of Vyvanse cannot help.  I am currently on two medications for depression, cymbalta and wellbutrin.  I also have central and obstructive sleep apnea and chronic neck pain.  

I spoke to my psychiatrist about possible drug side effects and she stated that it was not the anti-depressants.  She also has no idea what could be the cause.  

Could you please give me an idea what could be the problem?

Thanks!