If you are playing a MIM Fender bass, it could be static electricity from the pickguard. Especially true if your environment is dry. Heaters and fireplaces can really dry out the air in winter. To test, wipe down the pickguard with a used dryer "anti static cling" type sheet.
Acoustic owner's club - member #3, US Peavey Club Member #345, Grumpy Old Fart #77