I'm assuming you see the bruise on the outside of the cheek (?).
It's more likely she was playing with a toy and bumped it or her hand against her face (hopefully her toys are soft!). If it's the only bruise you see i think you're safe to just watch it fade. Check her for other bruises too. Sometimes bruises show up when a patient has a problem with their blood (clotting problem) or other reasons.