Unless the sites you looked at involved underaged participants, porn is perfectly legal, assuming that's what you mean by inappropriate. If they were underaged, unless you chatted on line and contracted for sex with someone who turned out to be an undercover officer, or downloaded photos and sent them to others, the odds are good that you won't get caught, especially since you were doing this only for a brief time and have stopped.
If you were looking at sites involving sex with underaged participants, you should absolutely not
call the FBI and confess to a sex offense unless you would like to be charged and risk a conviction and lifetime registration as a sex offender
. You should also keep this story to yourself, since you never know who would want to use it against you. If you werent on underaged sites, there is nothing to confess.
Keep in mind that if you downloaded any underaged files it might leave a trail if you ever had to bring your computer in to be debugged, and in most states, techs are required to report that. If you viewed but didn't download, erase your history. Do that while it's on your mind so that you don't forget it.
In the unlikely event you are contacted by authorities, admit to nothing. Just tell the police you have nothing to say until after you have spoken to a lawyer, and then go get one.