What causesBurning sensation inside my penis after ejaculation?

I am not sure. BUT burning can be a sign of a possible infection, so best you get evaluated. Good luck.
Many things. It could be anything from irritation to injury following vigorous masturbation or intercourse to infection. A little trial and error should help you out. If all else fails, see your doc for some testing.
Not unusual. This is not unusual and it usually just means that the urethra is irritated by the force of those ejaculations, especially if you ejaculate more than once or masturbate vigorously. If the burning persists more than a day or so it could indicate an STD and you should see a physician .