Bashing a tennis ball often leads to muscle imbalances - the bashing muscles that push forward get stronger and stronger with the bashing, while the "stopping muscles" don't.
The resultant "muscle imbalance" means laxity at the shoulder joint - and hence undue stress on non-elastic tendons and ligaments, resulting in sorenss.
You may want to consider keeping the muscles around your shoulder, elbow and wrist in great shape by doing the Thrower's Ten Exercises: http://www.muhlenberg.edu/pdf/main/a...throwers10.pdf
Also problems in technique could have crept into your serve during your period of limited play.
The two most common problems in technique involve a failure to "aim the chest up at the ball" and a failure to have great vertical shoulder-over-shoulder action - "You've got to drop the left shoulder".
These two problems are addressed in the following videos:
Your serve technique doing more harm than good? http://www.youtube.com/watch?v=GgdXawklcZk
Preventing Rotator Cuff Injury http://www.youtube.com/watch?v=lTRvxaBMh8s