This is easy to fix. Pokemon Go does this on purpose so that you think you need to buy increased size packs, so you spend money! You don't need that. Just follow the easy instructions in the video, or do this.
Step one: Open Pokemon Go
Step two: Open the Items Button
Step three: Look for potions that have over 35
Step four: Delete potions over the #35...you just don't need them.
Now...your pack is half empty. You are now a player. Congrats.