I don't think its the benadryl. And I would know, considering the 1200-1600 mg of the stuff I take every day. Its probably just the usual horrible anxiety stuff. Since when you get anxious, you subconsciously tense up everywhere. Your shoulder muscles are huge and are directly connected with your chest and back muscles, so physical strain tends to hit those areas at once.
You probably don't even notice you're doing it. A good way to find out is to try very hard to relax your upper body. If I'm right, you'll instantly know its muscle strain if you try this.