What Causes Pain And Pulling Sensation On The Right Side Of Leg?

I am a 54 y/o and postmenopausal. My health problems: HBP (controlled), had a Thyroidectomy on april, 2007 and I am currently taking 112 mg of Synthroid, TSH levels are normal. my main complaint: I was diagnosed with Advanced Degenerative Osteoarthritis in my right knee for approx. 2 years now. I will try to explain, as best as possible, what I've been feeling lately. For several times, out of the blue, I feel like the long tendon (?) on all the right side of my right leg gets pull, feel an pain and due to this I feel like if I walk I will fall. It doesn't last long, but it totally scares me.