I was doing pullups and the next day i started to feel a tightness and uncomfortable feeling in my right shoulder blade. what is the cause of this.

Strain/non dominant?: If it is ur non dominant hand, it may b weaker & was strained while ur dominant is stronger an could handle it if it's ur dominant u may have used it harder 2 do the pull ups, aiding your weaker side. Also depends on ur grip being equal on the bar. Does not sound more then usual stiffness after a workout. Something torn etc. Would cause immediate pain. Ice, tub soak nsaids rest i. ?Massage.