I saw this movie years ago (mid to late 2000's), maybe on HBO or something similar. All I can really remember is a scene where a girl with long dark hair, probably in her 20s, is seen submerging under water in a bathtub. Then there's a very sudden jump scare where she turns into a monster, then quickly cuts to the next scene. The glimpse of her as the monster is very short, but I remember it.

I seem to remember she was maybe possessed by something, which was caused this.

I have one other detail but I'm not 100% certain its from the same movie. What I remember is at one point the demon inside her was able to transport to other objects(?), or maybe just people.

I don't know. It had a a very 80s feel to it. I think the scene after the bathtub she may have also appeared fully nude in another room.

It is definitely not Nightmare on Elm Street, The Shining, or Slither (too new).