Off Season soreness

Recently I have started to step up my armstrength training with my pitching coach (whom I see on sundays). As a result of this I have been feeling soreness/stiffness in my shoulder. The soreness is basically in the general area of my rotator cuff but mainly in the infraspinatus/teres minor and the anter point of my deltoid on my shoulder. It doesn’t feel like something is wrong but I am just wondering if it is normal to be sore from training in the offseason or should i be increasing my workload capacity training at a slower pace?