So, I reboot in safe mode & could not replicate the problem (that took a load off my mind, phew!)
Then, did a clean boot to isolate which software was causing a conflict with my mouse. Turns out it is my printer - a Brother MFC-J5620DW model. Last wk, I started getting an alert that the yellow ink is about to run out - somehow this was causing a conflict in the background. Swapped out the yellow cartridge for a new one & problem solved!
I'll send an email to Brother customer support to give them a heads up about this.
Thanks to everyone for brainstorming this with me. And, a mighty big thanks to Userque for suggesting the safe mode option
Wow, dump that printer in the garbage and never look back.