My cat is getting what I think is leg cramps?

I am writing in regards to my cat Tony. He is a year old almost to the day. He is a very active but indoor only kitty and the other day he ran off the top of the sofa to land about five feet away right in front of me. As he landed I saw his back leg give out under him. He sat down and made a sound of pain and annoyance. This was the first time I had ever heard him make a sound like a growl. He laid down right away and I checked his leg for a breakage. He stretched it like it was cramping and was quiet and calm for the rest of the night and the past two days( it was not like him to be so quiet but I thought he was just resting his pulled muscle). His leg did not seem broken and for about two days every time he walked his leg seemed fine. Last night he did not sleep with me at the foot of my bed, something he has never ever done since the first day I brought him home. I was very concerned about his lethargy and change in behavior. Then this morning he went to jump on the bed(it is only about six inches off the ground) and as he got in the position to jump he started making the same sound and sat down very quickly. I got off the bed and went to him. He walked away from me very quickly to another room and I followed slowly. He laid down in the kitchen and stretched his leg again like it was cramping. Once again I checked for breaks and stretched out his leg he seemed a little unhappy when I touched the inside top of his leg but he did not jerk it away just growled a little. I sat next to him and pet his head to comfort him. After a few minutes he was walking again and then went to sleep sprawled out on the sofa. He still seemed like the leg was bothering him but not as bad. He was fine for about eight hours and even played with his toys for the first time in days as if nothing had ever happened. I thought he was feeling better but in the last two hours he has had two similar cramping incidents. What could be the cause of this? Has this happened to other cats before? What can I do to help him?