someone forged my signature?

Yes, that is completely illegal. I am a lawyer, and would take that person to court, or at least inform your supervisor.

Do you mean criminal? Depends. Was there an intent to defraud?

Fraud: the act of intentionally making a false representation of a material fact, with the intent to deceive, which is reasonably relied upon by another person to that person's detriment.

Of course it is.
a

No, you should bring it to the managements attention....