Get a labour lawyer, and get them on retainer. You may not need it, but that's fine. Lawyers hold funds in trust - not that anything can happen, but having them on retainer means that should you be in some sort of sticky situation, you can continue using said lawyer, as they're already on retainer.

Then - stop.

Don't do anything, anything, anything, without a discussion with said lawyer. Follow everything that lawyer says to a T - full stop. The only opinion that matters is the labour lawyer, who will undoubtedly base things on your contract, and any e-mails you exchanged prior to signing said contract.

Phone calls / voice conversations / things that aren't written down are too much of a he said/she said. Don't offer any money, don't so much as have a conversation with the company. For the time being, don't see your old co-workers, don't exchange email, just don't do anything.

tl;dr - Lawyer up