The law states that the employer may only consider felony or misdemeanor
convictions that have a direct impact on your ability to do the job. I don't know what the job is or what you were on probation for, so I don't know if that applies. Also, if they let you go because of your record, they are required to notify you in writing.
However, since they said that they didn't think you were a good fit for the job, they'll have a defense, because that falls within the permissible realm of at-will employment.
If you believe they violated your rights, you can contact one of the following agencies:
Regional EEOC Branches
Philadelphia District Office
21 S. Fifth Street, Suite 400
Philadelphia, PA 19106
215-440-2600 or 800-669-4000
TTY: 215-440-2610 or 800-669-6820
Pittsburgh Area Office
1001 Liberty Avenue. Suite 300
Pittsburgh, PA 15222
412-644-3444 or 800-669-4000
TTY: 412-644-2720 or 800-669-6820
Human Relations Commission