error A2022: instruction operands must be the same size

These two lines are your problem:

sub eax,my1337Sk1LLz    ;subtracts 1337h from usPop in eax
mov Difference, eax     ;stores eax into Difference

eax is 32 bits, but both my1337Sk1LLz and Difference are 16 bits.

There are two ways you might get around this:

1. Changing the size of my1337Sk1LLz and Difference. Right now you have the types as WORD and SWORD, respectively. You can change those to DWORD and SDWORD to make them 32-bit.
2. Zero-extending and truncating. You’ll need another register. I’ll use edx since you don’t seem to be using it there. First, you’ll need to sign-extend my1337Sk1LLz: movzx edx, my1337Sk1LLz ; move, zero-extended, my1337Sk1LLz into EDX Then you can do the subtraction: sub eax, edx ; they're the same size now so we can do this Then you can store the low word of eax into Difference, discarding the high word: mov Difference, ax