I Have This Sharp Pain In My Upper Left Back

I have this sharp pain in my upper left back and neck area. Ive had it for over 4 months and it hasn't gone away. Competitive sailing is my sport and i ask my coach what i could do to help it. He said that it could've been my posture and to stretch it. I have changed my posture and have been streaching it but it won't go away. what to i do?