Assuming all other factors are equal the tiredness is likely to come from the body reacting to the consistent increase in training load. Makes it even more important to recover (eat and sleep) well.
I tend to go through phases during training campaigns ranging from being highly motivated to knock out each session right down to cba to train at all. There is quite a delicate balance between knuckling down and making sure you do the sessions balanced against sometimes listening to your body and easing off to prevent injury/illness and a longer layoff.
I've currently hit a bit of a plateau after a successful base phase where I am struggling to motivate myself and fit in all the Fink sessions as it Builds. I'm now doing deals with myself about what is critical (long bikes and runs) versus every session being comlpeted.