Yes its normal. And you can check with the Doctor for any infections just to rule it out.
But the thing is, she is young. And even though a child is potty trained... they still have accidents.
So to a certain extent, you need to accept that.
Put it this way, even Kindergarten kids have accidents day or night. MANY Kinder kids also still wear diapers at night. Night-time bladder control, can take even until 7 years old, to develop. Normal. Per Pediatricians, and my Daughter's Teachers.
At 3 years old, this happens. It happened to my daughter at 3, as well. They think they have full bladder control, but don't or they wait until the last minute to go. The best thing is not to pressure them... because stress will only add to the problem. Or it will turn into a battle.
My daughter is now 7 years old. She's been potty trained since she was 2. She still once in awhile has accidents, just a little pee leakage because she sometimes waits too long before RUSHING to the toilet. Just recently as well, she had a pee accident at night. Normal.
Accidents do happen. "Regression" in a child is a symptom of stress or something they cannot cope with. Or, they are simply not in total control over their bladder impulses. Again, she is quite young, to be perfect about it. In preschool, when my daughter was that age... the Teachers know that pee accidents still happen. It does not matter how long the child has been potty trained.
Perhaps talk with her. Not pressuring, but asking her if she knows why it is happening? Or has anything in her life changed? This can cause accidents too....
all the best,
Susan