welcome to HCM!
resulting from slipping or sporting events is a common thing but few bumps on the forehead of this nature result in serious injury.
The forehead has an abundant blood supply and injury to forehead often results in bleeding under the skin. When the bleeding is restricted to one area, it causes bruising and swelling.known as a hematoma
.Even a minor head bump can cause a large swelling.
Since you had x-rays for your neck and no new symptom so this bruise
seems to be minor injury.However if any new symptom symptom like headache,vomiting
develops then you must see your doctor.
hope this will help you!take care!