I signed an agrmt not to sue in exchange for severance - can I still sue?

I signed an agreement not to sue in exchange for a severance package after being terminated for a very "fluffy" cause. My employer is playing games with my final paycheck saying they are going to withhold an entire month of health insurance premiums out of the final check, even though I know this is not standard practice for any other employee that has been terminated.

I was a top performer the entire time I worked there and my performance evaluations prove it. Employees that have received terrible reviews and have been counseled for attitude issues are still working there, yet I was terminated. The reason I was terminated was someone said I acted inappropriately when I was off of work and out for drinks with coworkers. The employees that complained are employees that hold a grudge because they have received disciplinary action from me as their supervisor. Two of my peers, also senior managers have been so drunk at company functions in they have actually fallen down stairs, pole danced, and taken some of their clothes off. No disciplinary action for them - they are both males. I am female.

Other employees were also given progressive discipline before being terminated. I was not. Again, I'm female.

I didn't want this to get ugly but honestly, this is getting ridiculous. Can I and should I sue them?