Why does it say my bag is full at a Pokestop when I have no Pokeballs left?

Solution 1:

The items are not limited in quantity, as long you have free space on your bag, so doesn't have to be pokeballs that will fill the bag. (Which means for example, Potions can fill the whole bag space, thus leaving no space for your other items like pokeballs.)

Your bag contains all inventory items, while pokemons that have been caught have a separate storage. Inventory items include:

• Pokeballs
• Potions
• A Camera
• Incubators
• Revives
• Incenses
• Berries
• Lure Modules

Each item will count as 1 space in your bag.

How to have more space for your bag?

1. Click the Trash Bin icon beside the item you want to remove, then input a quantity of that item you want to be removed

2. Upgrade your bag.

Solution 2:

Pokeballs are not the only item in your Inventory.

Potions, Revives, and Razz Berries also count towards your bag count.

To clear out your bag, you can tap the Pokeball and select your Bag. From there, hit the Trash Can to get rid of any excess items.