What Could Vibration In Knee With Enlarged Muscles Indicate?

I am 12 years old recently i have been feeling weird vibrations in my right foot my right knee and my diaphram area i havent found a way to stop them and they only happen once in a while and last for a couple of secounds but just today my knee was doing it for at least 30 minutes. When i felt my knee while it was vibrating i could feel the mucle getting bigger then smaller it felt like it had its own palse. I would like to know what is causing this and if i Should just ingnore it or if it is more serious. Thank you.