I think it's a universal phenomenon after someone starts any sort of new practice-- things start out breezy and wonderful, and then they get difficult.
When someone becomes a Born-Again Christian, for instance, it's an act of faith, but then it's immediately followed by a lot of Works -- living life differently and starting a bunch of new behaviors, and stopping old behaviors. It's common that new believers have all sorts of difficulties manifest themselves. Christians explain this by talking about the Devil not paying any mind to you when you are doomed to Hell, but when you rebuke the Devil and accept Jesus, then you are in Play, and the Devil is going to fight you. It's viewed as a good thing, because again, Satan wouldn't be paying you any mind and making your life difficult and tempting you if you hadn't thrown off his yoke and taken on the yoke of Christ instead.
When I practiced SGI/Nichiren Buddhism, they used a metaphor of turning on a faucet for the first time in a long time -- there's going to be a bunch of old junk in the pipes, and you turn the faucet on, and all this old junk comes out of the faucet. It's a natural result of turning on the faucet, and so rather than getting upset, you should be happy. It's proof things are starting to flow through the pipe. Right now, it's gross and smelly, but the cool and clean water you want is coming down the pipes. Just be patient.
Anyway. I seem to have a bunch of junk in my pipes or something. I have been tired after a full night's sleep, being more ADD than usual, and feeling more stressed than usual, and finding myself in funks for no good reason. I think it's partially because being Mindful more often means more noticing of how I am distracted or engaged in negative thinking. Then I'll distract myself from this noticing and then catch myself doing this. Etc. And there's a lot of negative emotions that come up for no reason. Noticing there's no reason is a good thing, but still. It's all stuff that makes it more challenging to be mindful enough to sit. I still need to develop a routine to practice every day, because my ADD-tendency makes me avoid this sort of discomfort rather than just sit with it.