Is it an at-will termination or retaliation based on a possible pretext?

I was recently terminated after 11 yrs w/ the company. My supervisor (of one year) sited job performance and punctuality as the reason. I was handed my negative performance eval. and told I was fired right on the spot! It was never brought to my attn. that my performance was an issue. I have accolades and prior evals. to back that. However, I did receive a written reprimand for the punctuality 3wks prior - which I corrected. My supervisor was inexperienced, didn't manage, oversee, direct or assign any of my work. It was directed by higher level mgrs. of the org. My supervisor even noted this in an email. I was surprised and confused by the actions but later remembered that a former coworker filed a discrimination charge w/ EEOC against the company and those same high-level mgrs. My name was mentioned in the claim as someone who was affected by discrimination. (I could also confirm what was alleged in the claim against them.) I just learned that the charge was being investigated at the same time my employer was "building a case" to get rid of me. Although I didn't actively participate in a legal proceeding, could this discrimination charge be the underlying reason why I was terminated? Do I have a case? What proof do I need?