Hello and welcome to HCM,
A fall on the buttock can lead to many types of injuries.
However, depending on the symptoms,
soft tissue injury is more likely.
Soft tissue injury commonly leads to bruising and/or
hematoma formation.
Hematoma formation occurs due to accumulation of blood in the soft tissues.
The collection of blood disappears spontaneously over a period of time.
The color changes from red to blue to green to yellow.
Pain can be experienced especially on pressure.
Soft tissue injury is the most probable cause of symptoms, however, a clinical assessment is required to rule out any other injury.
Thus, I would suggests you to consult your primary health provider for clinical assessment of the injury site to rule out any skin or any other kind of lesion.
Thanks and take care
Dr Shailja P Wahal