Company refusing to pay properly after quitting

Today I quit my job (they actually told me if I didn't like it to leave and never come back) With that being said I did leave. They then state that my 2 paychecks to come will drop from 12$ an hour to minimum wage (7.85-7.95?) Thats going to be a $300 difference each check. I do not believe I've ever signed a handbook (they didnt have on when i got hired) and was never told this. I just wanted to know if this is legal, or do I have grounds to stand on to fight this. They also changed our pay dates without notice, we have not been paid for almost a solid month.