Is it normal to have a knot under your breast bone? I have a knot right under my breast bone and at times its sore and I have troble breathing because of it.

Not. Not exactly sure what you are referring to as a knot. At the bottom of the breast bone (sternum) there is cartilage that hardens with age. It is called the xiphoid process. I don’t know if what you are describing as a knot is your xiphoid process. Xiphoid syndrome is when that area becomes painful. Does it hurt more after you eat a large meal or if you move in a particular way (such as lifting something or bending over)? Possibly, it hurts when you breath in and out. Sometimes the doctor doesn’t figure out a reason why it is hurting but sometimes it may due to other medical conditions; like problems with your heart or your gallbladder. On the other hand, maybe what you are feeling is a knot and not your xiphoid process. The best way to find out is to see your doctor and be examined.
Not. Not sure what you mean by a knot and not sure if you mean at the lower end of the breast bone or actually below it. But regardless, you are having trouble breathing and that is enough to warrant a visit to the doctor asap.