Hi and welcome to HCM
There are some very serious possibilities when a person has unexplained bruising, including various blood diseases and diabetes
This kind of unexplained bruising is usually nothing to be concerned about, but it is generally considered wise for people to have it checked out as a precaution.
When people engage in physically taxing sporting activities, they forget about minor physical impacts, and some of those impacts might be forceful enough to cause a bruise
I do hope that you have found something helpful and informative and I will be glad to answer any further query.