using a vibrogym

hi, im new here and wonder if someone can help me please.
im a full time wheelchair user. 28yrs ago i had a cluster of benign spinal tumors removed (2 ops) 1 tumor was a benign Astrocytoma. i was able to walk after 1st op but had a relapse after 2nd op and a bleed in the wound. i don't have any balance but can stand holding onto something secure, for a short while to transfer from w/chair to chair, bed, loo etc.
with my consultants permission for 2yrs i have 'excercised' using a vibration gym which i sit in front of in my w/chair and place my feet on the vibrating plate. im not allowed to sit on the plate and cant stand on it but ive slowly built up to 30/40 mins a day this helps my legs/lower back support me whilst standing/transfering. recently ive been feeling tightness in my lower back and legs causing uncomfortable tightness and spasams in my right hip and legs which interfere with me standing safely. when i don't use the gym for a few days this subsides but i dont feel im getting any stronger so im continuing with the gym hoping these symptoms will disappear. am i doing the right thing? once ive used the gym my legs and lower back are no longer tight and feel good but the next day the uncomfortable tightness/spasms return and standing to put myself back together again in the loo is becoming more wobberly . will these symptoms pass or am i doing too much on the 'gym'? please can someone advise me?