The only things I can come up with are a uti (urinary tract infection) but then i'd expect the burning to be with in the shaft and not only at ejaculation.
Or the muscular cover of the vas daferens going into spams causing the sensation of burning/itching.
I'll put my hands up on this one and say you got me stumped
first time for everything.
If it persists the best course is to see your doctor especially as it was a sudden change.
It could be related to puberty based on your age.
Don't worry or be scared to see your doctor s/he has seen 1000's of penises of various shapes and sizes.