What happens after reaching 99x items of a kind?

I know it is possible to get over 99 Balls.
I have 99x Max-Potions and haven't found one since 30+ Pokéstops. Which may answer my question or may be coincidence. I never recieved less than 3 items from a Pokéstop.

Imagine I keep 99 pieces of berries and each potion (don't mind the bag-limit ;) ) and I spin a Pokéstop.

Do I get only Pokéballs? Do I recieve only the items I can carry? Or will another "stack" of items be opened? Or do I have sheer bad luck with getting Max-Potions?

What happens after 99? 100. Seriously. I've had 100+ berries, and various potions.

So long as you have room in your bag it should be possible to get more*. I've gone a few dozen Pokestops without getting more Ultra Balls, and I have nowhere near 99 of those; they are just the rarest of the 3 Pokeball types. Max Potions, being the rarest of 4 would, I assume, have a lower drop rate than even Ultra Balls (rates may depend on your level).

*caveat: due to the nature of programming, it is possible that the item count would eventually overflow. But given bag limits, the amount of an item necessary to do so is likely beyond what is possible without otherwise breaking the game.

There is no limit at 99, at least for Ultra Balls. This must be just a co-incidence that you seem to have stopped receiving items when reaching 99.