At this age, you don't want to take any chances with this.
This could simply be arthritic changes that often come as the pet ages. If this is the case, anti-inflammatories can help a great deal and they are pretty cost effective.
Aside from that, I would worry more about a heart problem that is causing the weakness.
I think that you need to see the vet as soon as you can.
Let me know if you still have questions.