When you loot someone, or pick things up, the items that you don't have will be represented by numbers to the left of the item name. Items you have will be numbers on the right. So for example:
1 Painkillers 2
Means there are 3 painkillers, 1 on the ground and 2 in your inventory. Backpacks are basically the worst way I've ever seen anyone impliment backpacks in a game ever. If you're not looting someone, or picking something up on the floor (like if you just press G in an open field and click open backpack) then I'm pretty sure that it's the same as above, but the number on the left is what's in the backpack? It might be the number on the right... You should be able to work it out from what's in your inventory. When you move items around there you're moving them between your backpack and your normal inventory.
If you're looting a corpse, or picking something up from the ground, then the backpack works as a seperate inventory space, and it'll show you how many slots it has and what's inside it. Generally I try to only go in my backpack if it's like this, because it's slightly less of a clusterfuck due to being able to see what's going on.
If you try to move something out of the backpack, and you dont have room in your inv, theres a pretty big chance something will get deleted because the game is a buggy mess. (As a general rule, don't ever use the "take" command (take weapon for example) as if you dont have enough space to do it something will get deleted. And it's usually best to not move things directly into and out of your backpack if you can avoid it, as it could bug out and delete stuff from your inventory.
Though as a general rule the game will delete things on you all the time regardless because Arma 2's inventory system is horribly designed in general, and even worse when it's being used for a survival RPG sim.
As for the FPS issues, ARMA 2 is kind of a mess in general. Try messing around with the settings? I've heard that for some people windowed mode will drastically improve performance, and also that it's possible to make the game run worse by turning the graphics settings down to low, so I guess just mess around with it and see what you can do?