Comfortable but newfound shoulder pain

We purchased the Alexander Signature Series with high expectations. And while this is an extremely comfortable mattress, I don't sleep nearly as sound because of shoulder pain I experience throughout the night. I did not have this problem with my previous mattress. I'm a side sleeper and tend to sleep with my arm under my pillow and it causes numbing in my shoulder. Perhaps it'll take more time to get used to the mattress and slight adjustments to my sleeping habits because I really want this mattress to work.