Hey, welcome, stick around.
As explained above, phenibut has a long half-life, and apparently it also absorbs slowly, meaning that even after quite some time it will still be building in your system. The half-life of a drug means that half of the total amount affecting you will be eliminated in that amount of time. So, if you take a very high dose (of any drug), it will be around longer. Let's say you took the amount needed to affect you... in one half-life period (in this case ~10 hours), it will have dropped off enough that you won't feel it much, if at all. But if you take twice the amount needed to affect you, after 10 hours you will still have the amount needed to affect you, since half is eliminated, so it will take another 10 hours to fully disappear. If you take 4 times the amount needed, then after 10 hours you'll still have twice the amount left, and after another 10 hours you'll still have the amount needed to affect you, and 10 MORE hours after that it will be gone. So in this way, duration increases as the dose raises. This is how people can be affected by taking way too many psychedelics even days later, for example.
If you then add a slow absorption rate, it makes it even longer... your body can absorb things at a certain rate, not any faster. So if you take a bunch of phenibut, you will keep getting more into your bloodstream for a longer period of time, which means it won't start to drop off at all until later. Of course a lot of drugs absorb very quickly, so taking a larger dose makes it almost instantly stronger, but not all of them are like that.
I wouldn't expect that withdrawal would be part of it at all, instead it's just that it took way longer to get out of your system.