My daughter had her first day of daycare yesterday and now has these bruises on her leg. What can cause that?

http://ift.tt/2mE4WhS is 4 1/2 months old, and we have kept her at home for the whole time. We never noticed such bruises before. My wife has to start back to work, so we left my daughter at the daycare. When she came home, she had these marks on her leg. Any ideas about what could cause it, and mostly, should I be worried? via /r/Parenting http://ift.tt/2mpHzIu