Sounds like elastic deformation in action. You were probably in the middle ground between your original size and the size needed for it to become permanent. You’ll want to get plastic deformation if you want it to stay. Basically there are two types of gains. Temporary gains (elastic deformation) or permanent gains (plastic deformation). Temporary gains eventually become permanent if you keep going, if you stop too soon it won’t happen.
While having a temporary gain, caused by elastic deformation, every time you rest it slowly creeps back to it’s old size and given enough rest it will actually take you back to square one. This is not ideal, that is why frequency and consistency is of great importance in PE.
It’s harsh but I suspect one missed session can take you back several steps and perhaps even to square one.
I’m going to use an analogy to demonstrate my point. I suck at this so hopefully you can understand half of it.
It’s like trying to keep a fire burning. Every time you aren’t adding wood to it (no pun intended) it’s burning itself out, but it keeps going for a little while on what it’s got. The more wood you add, the longer it’ll last until you need to add some more. You have to consistently keep adding wood to it with the proper frequency/timing (if you do it to slowly it’ll die out). Sure, you get a little breathing time every time you’ve added wood but it won’t burn forever and if you for some reason cannot add wood to it (your 3 rest days) for, say a day, it’ll die out and you have to start all over if you haven’t already kept it burning for a certain amount of time. Lets say you need to keep it going for 1 day, if you fail and it dies out before that you have to start all over. If you make it, you’ll have success and can now take a break.
Ok, the wood adding is the PE training, the fire is your gains. The good news is, you won’t have to keep it burning forever, once you’ve reached a certain time the temporary gains will become permanent and you may stop and live happily with your new permanent gain or you can take a decon break and then do another round/cycle to add even more permanent gains. And so on..
That’s pretty much my outlook on gains so far.. Maybe it makes sense to you, maybe it doesn’t but good luck! :)