What Causes Constant Dull Pain On The Shoulder?

Hi. I started lifting weights about 6 months ago and it has been great until recently. I am 23 years old, 5'11 and 195 lbs. Haven't had any serious medical issues that relate to this. I have a dull nagging pain on the outside of my right arm, just below my shoulder... Pretty much where my shoulder connects to my tricep. I can't tell if it is my tricep that has the problem or my shouler. It hurts to extend my arm and raise it upward. This isn't serious, but it's been a week or two and it's not going away and I want to recover. Please help.