As a general rule, an amended pleading stating a new theory of liability relates back to the filing date of an earlier pleading if it is not based on a new or distinct transaction or occurrence. I don't recall ever seeing that rule applied to a claim under the Texas Whistleblower Act, and I don't know whether it would fit your case, but that is what you need to be looking at.
I agree with Mr. Homes. The relation back doctrine will likely allow you to raise the Whistleblower claim. The reported claim must be made to the appropriate law enforcement or governmental agency such as OSHA, Police,Sheriff or EPA etc.