It's a spin off of the 'baby blues' phenomenon new parents sometimes experience. Basically, bringing home a new dog - especially a puppy that needs extra time and attention compared to an adult dog - is an enormous change in a routine and life in general. You're suddenly doing potty breaks every 20 minutes, finding things you forgot to puppy-proof, interrupting your sleep schedule for potty breaks at best and a lot of fussing at worse, worrying about whether the puppy is getting enough food, sleep, play, etc...
Every new puppy is a different experience with different quirks and challenges, but they all eat up a lot more time and mental/physical energy than you expect! On top of that, they are babies and fragile, so it's common to worry about infectious diseases, if they're growing well and correctly, what their adult temperament will be like, etc. You don't know what's normal and what might be a sign something's wrong.
Basically it's a big and stressful adjustment, even if you really do enjoy having a puppy in the house and have raised puppies before. Some people handle it really well and it's no big deal, others find it gets easier with every new puppy, and still others will only buy/adopt older dogs so they can skip that puppy phase entirely. The best way to handle it, in my experience, is to be extremely realistic about your expectations with a new puppy. Don't expect things to happen on a strict schedule, or in a particular order, and try not to hold your puppy to expectations based on another dog you've had, or know, or have heard of on YouTube. Being able to be realistic and flexible in your goals and expectations goes a long way towards avoiding feeling too frustrated or overwhelmed, at least for me. It's definitely harder than it sounds though!
Puppy blues = that "What was I thinking, this is too hard, I'm in way over my head, and can I send him back?" feeling that you have when you aren't otherwise thoroughly enjoying him.
Simon really has been a good puppy in the two weeks (really? already?) I've had him. That said, his yapping and whining are enough to drive me bonkers, and if I never go out in the pouring rain with a desperate to pee puppy again, it will be too soon.