Rear deltoids

I am a mid distance freestyle swimmer that seems to suffer from pain in my rear deltoids and upper triceps. My recovery time from this pain is exceedingly slow, especially after long workouts. I essentially feel like my pull has no power behind it, and the soreness tends to linger for 3-4 days after certain workouts. Is this a normal area to have soreness in relation to a freestyle stroke? Do I need more warmup, cooldown, etc.?