Question ? Knot Under Left Breast. near The Bone ?

Hey I'm a 21 Yr old who is a manager . I do a lot of lifting and a lot of movement at my job. I didn't get my menstraul last month in (may). My doctor told me I have high levels of testerone and or I think it could be stress ? Lately for 4 days u had tenderness under my right breast and now I feel a knot. I'm haven't checked to see if it moved or not because when I research people say theirs float around . But all I know it's about he size of a nickel and tender. What could this be ?