No, as long as you're not having contact with anyone else...and not touching any containated surface, you're not likely to get an STD.
As for a pain, it may be "blue balls"...the feeling males get during high levels of sexual excitment without release. It's more an ache... also, too frequent of ejaculation before getting enough rest can produce the same ache.
Your weird feeling in the face and chest may be a sex flush... or your blood pressure spiking before you relax.
If you're still worried about this by the next time you visit the doctor, bring it up to them in private and get their advice.
Hi - dont be scared to talk about masturbation, it is quite anormal reaction and especially in pre-pubescant boys, have you got an older friend you can talk to, a counciller at school or something, some body you can trust and someone that will tell you the truth
You cant get STDs unless you have unprotected sexual intercourse with a partner,masturbation does not cause STDs
As for the weird feelings you experience, this could be called an orgasm, I think I would ask about the sharp pain down the shaft of the penis, to your Dr. youmay have a small blockage from the testes to the penis which doesnt allow semen to pass through, it wants to come out but may be blocked, so this I think needs to be attended to
Dont be embarrassed but go and talk to a professional about this pain, and I am sure everything will be okay
Dont be embarrassed about masturbation, even girls do it
Dont be scared, sex is a beautiful thing and should be enjoyed and if you find out what these small things are you can feel happier and not so worried
Hope this helps