The facts you describe suggest that you were an at-will employee
This means that you could quit your job at any time and there's nothing your employer could do about it.
That also means the reverse is true: your employer could terminate you at any time and for any reason (so long as it wasn't because of your race, age, gender, disability, national origin, religion, military veteran status, etc.) and there isn't a darn thing you can do about it.
An employer may dismiss an employee at any time for any reason; a good reason, a bad reason, or for no reason at all.
Now, does that seem to be unfair or unjust? You bet it does. However, it is not illegal. That is the very nature of "at-will" employment. You serve "at the will" (or whim?) of the employer.
Of course different people can disagree as to whether or not the punishment fit the alleged wrong in your situation. But, because you're an at-will employee, your employer doesn't have to even go there. They could have fired you because they didn't like which hand you used to lift your coffee cup. They could have fired you because they didn't like the shade of green socks you wore to work. They could do this because you were an "at-will" employee.
Now, if you can prove that you were discharged because of your race and/or gender, then you can file a complaint with your local EEOC office which can be found here: http://www.eeoc.gov/field/philadelphia/
Good luck and best wishes for better days ahead. I hope that you find this information to be helpful in better understanding where you stand and why. I also hope that you find this answer to be ACCEPTable!