You do not give a whole lot of information. But, since you developed the tenderness after some physical exertion, it is most likely due to one of the overuse syndromes, such as tendonitis, bursitis, or aggrevated degenerative joint disease (DJD).
Not being unable to straighten the arm (elbow) could mean that you have something going on: in the elbow joint itself (degenerative joint disease); have inflammation of the olecranon bursa on the tip of the elbow; have tendonitis of the triceps, biceps, or other muscle/tendon.
Without being able to examine you, it is not really possible to tell exactly what is causing your discomfort.
Most of the time, these problems respond to rest, change in biomechanics, some antiinflammatory medicine, and ice/heat. But, since it has been five weeks since it started, and you have not had a resolution of the problem, you might want to have your physician take a look at your arm.
Same problem. Cannot straighten my elbow the last 8 or so degrees without serious pain. It doesn't hurt otherwise, only when it is almost fully extended and gets to that 8 degree mark. It has been this way for about five years now. I hurt it pulling a shopping kart out of the coral: the wheel got stuck on a bar and my arm hyper extended past 180 degrees. They did an MRI and found nothing. Went through physical therapy and nothing. Another doctor said I may have tweaked a nerve. Anyway, I have to sleep with a pillow propping my arm up and it also affects me when I play sports or lift weights.