you're not the only one who feels like that from time to time. many people do. i do. its quite normal. they often say that a change is as good as a rest, and its quite true about bass playing too. sometimes i find myself strapping my bass on and just playing the songs that i already know out of instinct. sometimes it can be boredom...but its not boredom of bass playing. its because i'm being boring and lazy. sometimes you need to put that little extra in to get that littel extra out. what do you do when you're in a rut? answer: you try something new. variety is the spice of life. so when i strap on my bass and immediately launch into playing the songs that i already know, try something new. practice walking bass lines for an hour. have a coffee. practice scales for half an hour. have a coffee. try learning some new songs from your bass book. have a coffee. it always feels good to learn something new, so thats what you should try to do each day or each practice session instead of doing what feels comfortable(ie going over the songs that you already know). you get out what you put in.
let me know how you get on, and good luck