What about verbal agreements? They're a pain in the ass to enforce, but constitute fully legal contracts (at least in the USA).

Or accepting money from one of the other founders? That could make you an employee and then you don't need to sign anything for them to take your code.

Have you signed any contract? An employment contract would place ownership with the company. You've mentioned you've been compensated which doesn't sound good.

(Hence why people are advising you to stop working)

Go sick until you speak to a lawyer.