We had a problem with seeming happy during the day and then crying at night at about week 1.5- 3. As Jerry said, the first thing our vet had us do was rule out pain ... which the only way to do that was add in more (or back maybe in your case) pain medications and look for a cause if there is something to "chase" or further investigate on the vet's exam. 

After that, we worked with a veterinary behaviorist who told us that some older dogs can experience canine cognitive dysfunction (kind of like dog Alzheimers) after anesthesia. However, first she also insisted that we rule out pain (for us that meant increasing pain medications and getting imaging of his neck). Then, she gave us a sedative to help us get through the night (no one had slept in over a week at this time) and had us work on some dog brain exercises to help retrain his brain. We only needed a couple nights of sedative and we keep up with our brain exercises to try and keep him engaged. There is a medication for CCD but it does not mix well with another medication that Grover was already on, so we couldn't go that route. 

Just some of our experiences to consider discussing with your vet! Good luck <3 

Just wanted to add Brownie was on pain meds for three weeks and he still take rimiydi and CBD. If you do get CBD just make sure you get the kind that is full spectrum.