Bruises occur when the small blood vessels are damaged and the blood leaks into the surrounding tissues near the skin). The color of the bruise changes due to the length of time. It starts off red, then blue, then purple, then green.Within two (2) and sometimes three(3) weeks the area is healed and the bruise disappears.
Trauma to the area is the most common cause of bruising.
Bruising on the legs may be as a result of a blood clotting disorder, involving platelets or an infection, such as endocarditis ( heart infection). It may also be due to the veins not functioning or a deep venous thrombosis (blood clot in the calf area).
Now an allergic reaction has to be ruled out (insect bites) . Certain medicines such as aspirin and nsaids( ibuprofen) can make an individual more susceptible to bleeding.
Sometimes the body attacks it own blood vessels and cause damage ( lupus and rheumatoid arthritis).
A health professional should be consulted to do a proper investigation.

The bruises are on her legs only, as if she had walked into hard objects? Sleep-walking and epileptoid disorders should be ruled out. Has she had a concussion injury, or changed a relevant medication?