Spider identification

I've never seen this kind of spider before. I live in Pennsylvania. The spider, all around, was about the size of a quarter if not a little smaller. It's legs/mid section and head where a deep amber red/brown. It's head seemed disportionatly small compared to it's butt (abdominal? It's rear). The weirdest part about this spider was that it's rear was triangular. Starting out at the tip, connecting to the spider, and then widening to the other points. It was a perfect acute triangle. It was bright yellow and had what looked like 3 or 4 small depressions in it, reminding me of a bright yellow leather couch. On both sides of the triangle there where what I can only describe as fins (like on the rear if an old muscle car) starting as a siding and following both sides and eventually expanding past the top of it's rear, just like the ones on a muscle car. They where blood red in color, causing the yellow on the rear to stand out.

It was crawling up the door frame of my front door, I poked at it and it tried to flee by dropping/sliding down it's web. I grabbed it's silk still attached to it's rump and varied it over to the bushes.

What kind of spider was that?

What it sounds like to me is an Arrowhead Orb Weaver.

Here is a link with a photo:

http://www.insectidentification.org/inse…

Nice find!