If they used the term "at will" then you did not have a contract.
At will employment can legally be terminated at any time, with or without cause.
Now, the fact that they named a cause here, performance issues, helps with any discrimination claim that you would make.
You've certainly described some issues that suggest that gender could have played a part, but the more important facts would be to refute their claim of performance issues. When an employer gives a reason, and that reason can be proved false, then it appears that they are making things up....we call this a pretext for illegal discrimination.
So, the only way you're going to know if you have a claim is to actually file a complaint with the EEOC
. Allege that you were terminated based on performance, without any performance issues ever being raised to you. State that you worked with females that treated you poorly and appeared to be setting up their own, female-centric office. Challenge the employer to identify and then prove the alleged performance issues against you and if they can't, then you'll know the strength of your potential discrimination claim.
You don't need an attorney to go through the EEOC. They are free and tasked with figuring this sort of thing out, so that is the first step here.