My Freelance Contract Was Modified After I Signed It And The Company Is Now Enforcing Terms I Never Agreed To.

Location: Texas.
I do freelance motion design work and in November I signed a short-term contract with a mid-size marketing agency for a six-week project. Standard stuff: rate, deliverables, timeline, a clause saying all assets become their IP upon final payment. I kept a PDF of the signed contract. Last week they came back to me claiming I had also agreed to a non-compete clause that prevents me from working with any of their direct clients for twelve months. I have no memory of that clause being in the document and when I checked my saved copy, it is not there.

I asked them to send me their version of the contract. The document they sent is a PDF that looks identical to mine on every page except page four, where the non-compete language appears in the same font and formatting as everything else. Both documents show the same signature date. I don't know if they edited the file after execution or if somehow I signed a version I didn't fully read, but I am almost certain that page was not in my copy because I specifically remember checking the IP clause on page four and there was nothing below it.

The reason this matters right now is that one of their clients reached out to me directly last month for a separate project, I accepted, and the agency is now threatening legal action saying I breached the contract. The client is a small local firm and the project was compeltely unrelated to the work I did for the agency. Do I have any recourse here if I can show the two versions of the document are different? Is a post-signature modification even enforceble in Texas?