Obviously, just because the temp agency employer (?) or client's company doctor says X doesn't mean much.
Even if you don't have health insurance, that wouldn't prevent you from seeking medical advice about X beyond the company doctor (who naturally works for the company).
"I return to the work comp doctor ..."There's a difference between the "company doctor" and "the work comp doctor" so you may want to clarify.A client's free to decide that they don't need you anymore or don't want to get caught up in you asserting a w/c claim against your temp agency employer (we can't know what the contract between them states). Unclear why the client believes it would be on the hook for a temp agency worker's workers' comp claim, but for some weird contract provision.
The message boards aren't the place to look for an attorney. While it might be unlawful for your temp agency employer to let you go for filing a workers' comp claim against it, it doesn't sound like this was ever filed as such (and if this client is basically their only client, they aren't in a position to dictate to their client who the client will and won't let work for them).