I am by no means an expert, when it comes to this, seeing as how I have only been in the iron game for about 3 years now, but over the last year I have really made it a priority to listen to how my body feels.
As far as joints and programming go, I am not sure what you're split looks like,but if you are placing significant stress on a joint too frequently, pain can arise. Last week I had to move my back day back a day, to the day before I trained chest, and I definitely felt like my training was not 100% on chest day, due to some slight shoulder irritation.
I feel like there are many factors that lead to lack of drive at the gym. From the information provided, or lack thereof, you could just need a little mental break from the gym, give yourself a few days away from it to rekindle that flame you know. Additionally, it is flu season, and I have noticed that my first signs of getting the flu/sick is a slight decrease in performance at the gym and appetite. Additionally, eat more and sleep more and see how that goes.
Just me .02