Rear Leg Spasms

Recently I've noticed my (approx) 3 year old female cat has had what I can only describe as cramp like spasms that mainly affect her rear legs.  She doesn't appear to be in any pain or discomfort and can get around albeit slowly.  This only lasts for a few minutes but I'm concerned that it happens when she is out and is unable to make an escape from dogs, cars etc. 

Any advice would be greatly appreciated please.

The are many potential causes for this,  from seizure related to spinal disc issues, or other causes.  If she is onyl getting around slowly it suggests discomfort or a lack of mobility.  No one can diagnosis this remotely, she needs a vet.  The issue is more than just her escaping from cars and dogs which can be hard enough with 4 working legs, but many medical problems can be treated more easily and with less $$ if dealt with sooner rather than later.