This explanation is given in Harry potter wikia
escape from capture at Hogwarts, Peter Pettigrew(Wormtail) had no choice but to rejoin
his master, or Sirius and Remus would surely kill him. He went to
retrieve Voldemort's wand, then travelled to Albania where he learned
from the rats there of a place in the forest where the creatures
feared to travel. They said a dark shadow (Voldemort) took over the
animals and killed them.
On his way to his master, Peter ran into Bertha Jorkins, a Ministry
witch who was on holiday in Albania, while she had stopped at an inn
for food. Although she recognised him, Pettigrew was able to lure her
into the forest and overpower her, before handing her over to the Dark
Lord. Under torture, Jorkins gave them information that helped
Voldemort make plans for an attack on Harry Potter, including
information on the Triwizard Tournament and the location of a faithful
Death Eater. Voldemort's rudimentary body.
Pettigrew milked venom from Voldemort's snake, Nagini, to strengthen
him and provide him with a rudimentary body. He carried Voldemort's
weak, small body on the long journey back to Britain, where they took
up temporary residence in the Riddle House outside of Little
Hangleton. Voldemort called Peter by his Marauder name, "Wormtail."
Over time, Pettigrew tried unsuccessfully to convince Voldemort to use
a wizard other than Harry Potter to rebuild his body, but Voldemort
saw this as an attempt at desertion and claimed that Pettigrew was
regretting ever returning to him in the first place.
There is no explanation of this in movies but are there in novel series.