Ever use a foam roller? They work wonders, I swear. Most gyms should have them on-hand, and you can typically find them in big box stores like Target and Walmart. They really help to keep the muscles in your legs loose, long and limber. And obviously you can use them on other parts of your body. But seriously, I'm a firm believer that every runner should be using a foam roller on their IT band...at the very least.
I always tell my students to push from the hips and squeeze the glutes to facilitate movement (swing from the hips, not the shoulders)...sounds like you're doing that. So try to get a feel for what your arms are doing when you're swinging. Are they relaxed? Perhaps you're a little uptight throughout the movement. Let momentum kick in a little to facilitate a smoother swing...but not too much as you don't want to take away from the exercise. Also, are you using a bell that's too heavy? And are you lifting the bell to shoulder height or higher? I advise my students to stick to shoulder height, maybe slightly above. Some swing it over the head which makes me nervous. My last thought: Perhaps you're just getting a really great shoulder workout! Maybe swings challenge your delts more than any exercise you've been doing. :-)