If it is a dull kind of pain it could be some inflammtion or arthritis, and possibly related to a past injury, if it is more of a sharper pain when you lift something then it is most likely muscle related. If you have been taking the Naproxin and it has not seemed to help then it is most likely a severe enough strain that you need prescription anti-inflammatory medication. You need to also use some heat therapy and try some Ibuprophen as well. If this has seemed to worsen and has lasted for more then 3-5 days then you may need to be seen.

It is possible that you may have tennis elbow, or tendonitis if it is right below the elbow as they can continue to grow worse over time and not see relief at all. The recommeded treatments are physical therapy, pain medication, immobility, and anti-inflammatory medication. If it has gone on this long this sounds more like one of the above.