I've got some white creamy stuff in my mouth (with brown coating on tongue), a drip at the back of my throat and abdominal pain. Stressing. What is it?

See doctor. You may be having thrush ( fungal infection ) in your mouth and spreading in to throat and stomach ,causing abdominal pain . Let your doctor examine and decide about the treatment .
Infection. Tough to say and we really cannot diagnose you over the computer. Sounds like some sort of infectious process, be it fungal-like an oral thrush, or bacterial. See the Dr, for an evaluation and diagnosis, so you can be treated appropriately. Good luck. (HealthTap Prime may also be able to help, where a Dr. can video consult you and may be able to diagnose and treat you.)
Need physical exam. These symptoms & signs may be totally unrelated, or, possibly part of a constellation reflective of a certain illness. At this point, it is a guessing game. What part of your abdomen? Upper right side? Epstein Barr will cause you liver pain, a sore throat with white spots on your tonsils, fatigue, fevers & chills. You may have post nasal drip. Brush your tongue with toothpaste, gargle & get exam.