Good Evening. I've Been Experiencing What I Thought Was A

Good evening. I've been experiencing what I thought was a pulled muscle for about a week now. The strain is under my left underarm extending up into my left breast (but only partially) and I can feel it when I take deep breaths or tense up my left shoulder and breast muscles. It's not unbearable by any means, I'm just concerned because it hasn't gone away. Otherwise, I feel fine.