Your "talk to a lawyer asap" advice was perfect. Stop there.

In the US, I've worked with the team at https://www.optimalcounsel.com/ before. They're great and founder-focused so generally get it and are reasonable on pricing.

Before you talk to an attorney, make sure you have your agreements together. That is likley your initial employment agreement, any grant docs, your separation docs, and the current agreement they want you to sign.

And yes, don't wait. If you need to take action, you may need to have time to ship docs, send wire transfers, etc.