red mark between armpit and breast

hi, it is probably nothing;
if u research skin red spots u will find several dozen possibles; but a important possible;
Blood Spots
Blood spots are formed under the skin but eventually appear on its surface. Blood spots are also called as purpura or petechiae and are a result of infection, bleeding or clotting disorders such as vasculitis. Certain blood thinning medications and drugs like aspirin also give rise to blood spots. Blood spots should be closely monitored as they may be a symptom of leukemia, lupus or cirrhosis.
best wishes