Hi Mesea,
You might be experiencing something called spasticity which is pretty common with MS. It is an involuntary stiffness or spasms in the muscles and can vary in intensity. It can probably wait til you see your doc on thursday unless its something that is very troublesome or severe.
Glad to hear you are starting Avonex! Please let us know how that goes and I do hope you start to feel better soon.
Kimber