I woke up this morning with 3 tiny purple bruises on my upper inner arm, they are about 2finger spaces apart from eachother and

I am female and 21 years old

I woke up this morning with 3 tiny purple bruises on my upper inner right arm, they are about 2 finger spaces apart from eachother and I have no pain. Could you tell me what could have caused this please?

I have a slight feeling of uncomfortable under my right arm pit but i have checked for bruising or lumps under but nothing to be seen ar felt.