problem

This seems to be happening very often. As i am playing i suddenly stop to be able to open my bag. Or if i open it with "B" i cannot use my items in it. When i kill a beast and open it can't get what i need from it. Why does it happen?
Help.

It's probably an addon causing the issue.
Try to disable ur addons and then enable them 1-by-1 to see which is causing it.
You can try with /reloadui the next time you encounter this issue and see if it's working.

i tried reloading but it didn't work. i even tried with logging out and then logging in again but the problem was still there.

What are actually addons ? im not really clear with them, im new.

Try deleting your cache. Also, addons are just things that can augment your game experience. Such as Bartender, for better action bars, or Deadly Boss Mods for assistance and notifications during encounters.

Sometimes I have such problem with Dungeon Journal at Cata. Maybe deleting the cache folder will help.

Thank you everyone!