Most likely it is a tear or rupture in the pectoralis major. If you are someone that lifts weights quite a bit, it probably happened during bench pressing. However it can occur in sports as well if you are pushing off the ground or a similar movement. Do you recall a sharp pain you felt during sports or Weightlifting?
There can be a lot of reasons why you are bruising, outside of being bumped or hit. For one thing, bruising is a sign of anemia. If you are going around with low iron, sometimes bruises will come up with no injury. Most people do not generally bruise for no reason at all. If you continue to keep bruising, bring it up with your doctor. He can usually do a quick set of labs to find out the reason. Sometimes it is something serious, but usually it is something easily managable.
Some signs of abuse are Bruises on the back, the back of your legs and arms, and the inside of your upper arms. Also, be careful if a child has marks on them and won't tell you where they're from.
I woke up with a rash over my whole body. Later there appears to be bruising and my arms and legs hurt like they are bruised.
possibly eczema
rash on upper back arms
The triceps are the large muscles on the back of upper arms.
Brushing by an object, running into furniture, are the major causes of bruising on the stomach. Otherwise, being hit with enough force. Generally, more bruises occur on the arms, legs, and shins than anywhere else. Head bumps can also cause bruising.
The upper arm is part of an arm and therefore there are no complete arms in the upper arm.
extreme tenderness of skin on arms. cuts and bruises easily
Arms
the upper extremities are your arms.
Thighs are where large muscles are, but they are also where many people (especially women) store fat in their bodies.
Arms