What is the legal age of consent in Pennsylvania and what does it mean?

In Pennsylvania the age of consent is 16.  That is the age a person must be to legally consent to have sex.  So in your case I don't think that a crime as been committed here.  Even if your girlfriend was 15 when you stared having sex (technically under the age of consent) there is still some protection in the law for you.  The crime of statutory rape (or your state's equivalent) can be brought against someone for having sex with another under the age of 16 (but older than 13) if they are 4 or more years older.  Since you are only 2 years older you cannot be charged with statutory rape.  Although, in that case, possibly a lesser criminal charge could be brought against you.  At this point, you should speak to an attorney in your area as to all of this.