Only one bruise is usually not considered pathological. Multiple and repetitive bruises covering larger areas need to be examined and treated. Since you reported that you are not on blood thinners, it is likely that the most probable cause for the bruise is impact, which is the most frequent too. Easy bruising is usually caused by blunt force on the skin that damages the connective tissue beneath.
In certain cases the capillary walls are more susceptible to excess force. Sometimes it might be caused by lack of vitamin C. In other cases weak capillary walls are caused by pathological disorders. Blood coagulation disorders might be a possible cause for small spot-like bleeding. This bleeding resembles rush that is caused by blood. This type of bleeding usually does not produce bruises.
Medications that affect the blood coagulation might be a cause for bruises more frequent than usual.
It is possible that the pain in the neck, arms and shoulders might be caused by cervical spondylosis. Cervical spondylosis is a condition in which there is a spine deformation in the neck area. This deformation is caused by a degenerative disorder of the intervertebral joints. Local anti-inflammatory creams might be helpful to ease the possible inflammation of the surrounding tissue. Physical therapy is recommended, too. But first, you are advised to confirm or to rule out cervical spondylosis by visiting a neurologist or an orthopedist to check for other conditions that might be causing the same or similar symptoms.