A bit of a long shot here, but this caused me significant head-scratching a while ago.
Is the mouse 'Optical'? ie does it use rotating wheels inside and interruptor gismos to detect motion? Mine is, and if a strongish light falls on it, it reacts as yours did, as the gismos get confused.
Solution:
1) Paint the inside of the cover with black paint
or
2) Draw the curtains/reduce the light