I know I am late on this....but if you are using the same technique as you always have, and didn't do anything different.....It could be tendonitis from not only drumming but other things in life. If so, you would want to take an anti inflammatory, and give your arms rest. If you ever ran track and had shin splints, its the same thing.
Another possibility could also be thatyou are simply building muscle. WHen was the last time you played that much in a day? If it has ben a while, you may have lost some of that muscle and endurance. You body may repairing the tissue much like after a good weight session.