well your dentist is your best bet, as a doctor would probably only tell you to go there anyhow, lol.
it may be that you just have a gum infection of some kind. but as u and i know there is only 1 person to be sure of that. i am sure you will be fine, and you are over anxious and worried, then as i know all too well, everything seems worse, lol.
as for teeth going brown, this can happen for many different reasons
smoking etc, etc
talk everything over tomorrow, and i am sure you will go home with a big weight off your shoulders
keep me updated.
sorry ive not gave any real causes, but i hope its helped slightly