Hi Emirnazim,
1- POIS sure can cause both extreme fatigue, and higher need for sleep than usual. Both are connected, of course. But it seems that POIS can resist a couple of good night sleep. Usually, when we are very tired, we have a good night sleep and wake up ok. But it is possible in POIS that it takes more than ne good night sleep to recover.
2- Volume of semen do not seem to be a factor. Some even have POIS symptoms with a few drops of pre-cum or of semen. However, it may have an influence in your case, because no two POISers have exactly the same pattern. Just try to notice if your symptoms are always worth when you have a higher volume of semen. In my personal case, the quantity of semen has no effect. However some say that if they have a more intense orgasm, POIS will be worst, so if higher volume = more intense O for you, it may have an effect ( Note that for some, it is the opposite - if O is better, POIS is less intense)