Being that you are not a salaried employee, you cannot be expected to work for free. This is the sort of thing that should have been worked out in the contract prior to starting. What did the contract say?

If you're not willing to work for "free", then when your contract is up, you can leave or negotiate a new contract stipulating paid overtime.

Your in a tough spot but depending on how much "overtime" your working, you really could be taking a big hit.

Steve