What the heck did I do to my shoulder?

This simply does not make sense. I ran a marathon...this activity is predominantly strenuous on your lower body, so why the heck does my right shoulder hurt? I've always had weird issues with my shoulder, including a bout of tendintis ten years ago when I was on the drill team in high school, and what we think is torn cartilage from "circus school" at the resort we honeymooned at three years ago in the Bahamas. Seriously, there was a circus school on the beach in front of the resort every afternoon ...and I love me some trapezee acrobatics.
Anyway, long story short, my right rotator cuff and biceps have been hurting pretty much constantly since last Sunday afternoon. I'm sure I've aggravated it some with work all week, as I've got a crapload of low back and neck pain patients right now (meaning I'm doing lots of manual work with this population), and I've been extra sore every night after working all day.
I'm going to give it until this Friday, and if it's still not better I'll break down and see the doc. (I'm one of those people...you know the ones that work in healthcare but don't ever think they need to see the doctor.)
Now that we've discussed my shoulder in great deal (and thus you've drawn the conclusion that I won't be lifting weights today...oh darn.), let's move on to other topics. Such as my run today. Today I went to Creve Coeur Lake park. This park is one of the hidden gems in the St. Louis area. It has a huge lake, which they do not allow motorized boats on, only sailboats and row boats. There are several trails to choose from, most of which are paved, but one of which is gravel. The trail I chose runs around the perimeter of the lake and is very pretty. This weekend was supposed to be the peak of the fall color in this area, and fortunately the weather here has been beautiful all weekend. I decided that I would do a nice three miler at a comfortable pace, which surprisingly enough was just over an 8:00 mile for me today. Where were these legs last weekend?
I took my husband to the airport this morning. He's headed back to Chicago for business for the entire week, so I'll be on my own all week with our dog and my parents' dog. Fortunately I've got a busy week planned, so it should go fairly quickly.