Many questions first:
Are there any scans at all?
Where is the location of the haematoma exactly? Anterior aspect over the tibial tuberosity?
Is there actual bruising over the soft tissue - ie yellow/purple/green bruising like it was knocked?
Does resisted quads cause pain?
Flat feet?
Any problems with the quads/patella tendon, or infrapatella fat pad?
Is it truly spontaneous, as in not associated with a particular activity, running/walking/steps etc?

How old is she? If <14-15yo, then consider Osgood-Schlatters.
Otherwise tendon or PFJ problem, or stress fracture.
If not associated with mechanical loading then perhaps infection.