Check her eyes, if it's dilated then she's on something, if she's breathing irratically then that too can mean she's on something. If it's not then she's mentally out of it.
I speak from experience because my ex used to do stuff like this (not peeing on chair, but similar aloof inexplicable stuff that made no sense) and she would be having a psychotic break whenever she would do something like that, as she suffers from atleast BPD at the minimum (never got diagnosed while we were together, she left midway through he treatment). But once she had an altered mental state due to Hypernatremia (low sodium levels, cause she wasn't eating much and drinking loads of water).
Not saying your GF is having a psychotic break, the likelihood is not high, but odd behaviour can show up even due to other mental health issues unrelated to BPD/Bipolar/schizophrenia etc.
But yea check her eyes first.