What you've got is inflammation. Sometimes referred to as tennis elbow.
Last year mine started in my elbow, went up to my shoulder and then down into my forearm. So the entire right arm/shoulder was really bad. So bad it felt like the bone had been fractured.
Ibuprofen fights inflammation so that's the first thing to take. The long term answer is a daily dose of Glucosamine Chondroitin and Fish Oil. Don't laugh. I got on this stuff in November of last year and have gotten through six months of heavy mowing/trimming with no serious problems. Not related to tennis elbow, but I also take a good daily vitamin and a garlic tablet.
Depending on you condition it should take from two to four weeks for the GC and fish oil to eliminate the pain.