After I do push-ups or I exercise a lot, my muscle kind of contracte itself, they twitch without me contracting them. Is that normal? What could cause that ?

After any excercise you need to do streches to help relax the muscles. You will get rid of any charlie horses, twitching or any other problems you maybe having. Keep up on the good work and don't give up on those exercises!

I've had this happen to me in my leg muscles. It freaked me out and first but then I got used to it. I don't think it is really a problem but you might want to make sure you are hydrated and to stretch.

In addition to stretching and drinking water, if the twitching continues, eat a banana. The potassium is good for muscle twitching.