A muscle mass at the back of the neck is this signs of arterial blockage or stroke?been to cardio and rehabilitation all say it was normal

Highly unlikely: 31yr old women don' t get strokes and arterial blockages for no reason. It could be a muscle spasm or trigger point; sometimes people have lipomas that can be mistaken for masses. See your doctor.

Not vascular: The mass you describe may be the soft tissue overlying the C7 spinous process (vertebra prominens). Many people have a hump there (in the midline). Blood vessels run in the anterior neck, and blockage/stroke risk etc will not present as back of neck mass.