I opt for the truth too. something as simple as an allergy or a misquito bite will cause bruising. When I am anemic I get bruises just by washing my face or sitting in one position too long. When Im anemic I can rest my chin on my hand for maybe a couple minutes and my chin has a bruise print of the palm of my hand complete with finger imprint on my cheek. not every briuse in life has an explaination. its when people try to explain the unexplainable that they tend to look guilty because they go above and beyond trying to construct a lie. therapy professionals are treained to see through lies of all sorts for example did you know the brain has an automatic lie detector of the eyes look to one side if the person is lying and the other if they are honestly searching their memory to tell about something. I found that out when I my past lawyer was telling me about lie detector machine testing.

Just go with the truth that way your body language doesn't ring the ok Im lying through my teeth here because I can't explain this. By being yourself and being honest too later on you won't get caught in some elaborate lie about this.