She may just have... anxiety about it.
OR, not want to have any pee accidents in bed... because she is not ready yet... to stay dry ALL night. But she doesn't want to make a 'mistake' and have a pee accident... and then she is then scolded for it or something.
Night time dryness, is actually something that is not attained, BIOLOGICALLY until even 7 years old. Until then, MANY even older kids, still need/wear a night time diaper, to bed.
When I was young... I in my mind, would think I always had to go pee before bed... or BEFORE I actually fell asleep... because, I did NOT want to have to get up to go pee, during the middle of the night, because I was scared to. So... it would take me a long time to fall asleep... thinking that one-more-time.... if I go pee... then I would not have to, get up/wake up during the middle of the night to go pee.
I was NOT doing it... on purpose.
Your child is so young.... at this age, 'impulse-control' is not even fully developed yet, and it is certainly not fully at-will yet. Nor cognition, nor at night.
A child this age... WILL HAVE pee accidents at night... it is childhood. It is normal... and certainly in a child... this young.
As I said, daytime and night-time pottying are 2 different things... and 2 different time-lines. Night time dryness.... is not yet, developed at this age, nor fully, nor perfectly.
My daughter, when even 7 years old, had pee accidents. NO biggie. It was not on purpose.
I simply use, with both my kids, waterproof bed-pads which makes clean up super easy.
all the best,
Susan