Right arm pain

i had my frame custom fit for me-a Calfee Tetra Custom that a 49 x 49 (yes, I am short at 5'3") and despite the custom fit and set up, after riding about 20 - 25 miles, I begin to suffer from unusual tightness in my right arm when. This doesn't occur at all with my left arm. The pain is right above the elbow -- perhaps a tendon. The pain goes away with stretching, thus sometimes I must take a break to stretch it and that usually helps. I try to keep my shoulders relaxed. I find it strange the the tightness is on one arm, although I of course do the majority of my braking and shifting on this arm.

Since I am trying to do longer rides (60 and over), this is a real drag.

Any suggestions?

Patricia