Crutches

I’ve been using (elbow/forearm) crutches for about 3 weeks after injuring my knee. For the past week I’ve had a really wierd sensation in my left hand – my fingers feel really numb, and I can’t move them properly. I’m assuming that this is down to using crutches. Does anyone have any experience of this? Is it serious? and is there anything I can do to stop it?

Thanks