A Client changed my contract without my knowing. I want to know the legality of this.?

Here's the short story. I emailed a contract to a client via pdf form. They took it upon themselves to change the wording of my contract and p it off as my unchanged contract at our meeting. I signed a copy for them without knowing of the changes. The client then mentions the changes later in a phone call. Now they have a contract with my signature agreeing to terms I had no idea were altered. I know this is unethical, but what does the law say about this? I am in Texas.