You were there when everything exploded. You saw the scene of how Bill entered Stanley's mind disguised as Stanford and then everything collapsed The town went back to "normal" after that but the Pines twins had fainted from the shock of the explosion and so had you so they took them to the hospital

Luckily for you...the whole town went to visit the Pines twins and no one went to see you, including your parents You were alone on the hospital gurney while in the next room you heard everyone celebrating Dipper and Mabel

Stanford was doing some quick tests on Bill's statue to make sure there weren't any dimensional rifts and that's why he arrived a little late. But he went to the wrong room

"Kids! I'm so pro-"

His voice trailed off when, instead of seeing his nephews, he saw you