Concerned!

Alright, so today I had my fourth Dr. appointment, and all was going well, heard the heart beat, gained very little weight (9 lb through my whole pregnancy so far which is super), blood pressure great, and then my Dr. measured my uterus and to her surprise, I have something like an entire new uterus from my belly button to my chest! she said that should happen around week 30 which is 10 weeks from now! and we know I am for sure 20 weeks cause I just had my ultra sound two weeks ago!
Dr. said it feels as if I have my normal size uterus, then it end right around my belly button, and something else starts! she has no clue what it is, the ultra sound and the gall bladder ultra sound did not show any signs of anything unusual, anyways, I have another ultra sound this Friday followed by a Dr. appointment so we find out what that is up there!
The Dr. had the nurse measure me, then the intern, then she herself did it again, she had such a confused look on her face! In general it is not very comforting to see your Dr. with that look on their face while they examine you, but on my way out she said "don't worry, it'll be fine"! YEAH, RIGHT! like I can sit back a whole week with no worry in mind, after all, it could be twins, a second uterus, or something weirder!
Wish me luck.