Could definitely be physical/medical - any of the following:
* symptoms of a food allergy or other allergen exposure
* urinary tract infection (is she experiencing any pain while urinating?)
* eating lots and lots of melon or caffeinated beverages, resulting in having to pee a lot more often or unexpectedly?
* is she constipated?
* big growth spurt or big developmental phase... is she sleeping a lot more or eating a lot more? or is she concentrating on learning how to (for example) skip or gallup or snap her fingers (etc.)? When big things like that happen, there can be a temporary ignoring of typical body signals.
Hope you're able to track it down quickly and/or that it resolves itself just as quickly as it showed up!