Hello, Thanks for your query. Unexplained bruising if occurs should be investigated thoroughly since it can indicate an underlying medical condition or ailment. Thinner people are more prone to bruising as they lack the fat cushion that helps prevent injury. It may also be due to deficiency of vitamin C, vitamin B12, folic acid and vitamin K. Many times taking medications like aspirin, ibuprofen, non steroidal anti-inflammatory drugs, and even birth control pills increases chances of sudden bruising. Topical and systemic corticosteroids cause skin thinning, making one prone to bruises. Many a time bruising can be caused due to liver ailments, kidney diseases and blood disorder. I do hope that you have found something helpful and I will be glad to answer any further query. Take care