Incorrect peng will cause discomfort in the shoulder joint.
If there is a tangible sense of mild pain, it must not be ignored.
Poor bodily awareness and an insensitive nervous system can mean that an inexperienced student may not even feel discomfort initially.
Taijiquan does not cause pain in the body.
Any indication of pain is caused by a prior health-problem or incorrect body use.
Correct your posture repeatedly until discomfort is removed.