Have Extreme Difficulty & Pain In My Legs After Sitting

Have extreme difficulty & pain in my legs after sitting or sleeping...when trying to stand my legs buckle & I lose balance. Additionally, I have fallen or tripped,losing my balance a number of times. I initially excused it as being "clumsy". Now however, I am experience the aforementioned problem. I attribute it to my muscles, although I have arthritis & had bi-lateral knee replacements 11 year ago this month. When this pain comes & weakeness comes, I have difficulty walking & standing. After a short period of time...I am totally better. I have also gained weight and have not been exercising or walking much. Thank you.
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