I Am 62 Years Old In Pretty Good Health. I

I am 62 years old in pretty good health. I mowed my yard last week. After I mowed, the next day my left inside knee has hurt. After I walk it kind of stops. When I sit and get up, it takes a while to quit hurting and able to walk right. It seems to be going down in my leg also. Not bad, just a mild pain. What do you think it is?