Brief Answer:
Blood collection caused by veins rupture.

Detailed Answer:
Hello!

Thank you for the query.

Can you please attach a picture of it so I can give more precise answer? When was this fall? Do you take any anticoagulants?

The lump is most likely a hematoma - blood collection in the soft tissues. Please note that due to injury veins have been ruptured so the blood could appear outside the vessels. Depending of the factors like injury strength, vessels size and vessels condition, blood coagulation ability, the hematoma can be large or small.
Small hematoma does not need any medical attention.
Large needs to be consulted with general surgeon. It may get infected as blood contains glucose (glucose allows bacteria to grow). Infected hematoma turns into an abscess.
If you have pain in this area, fractures should be also ruled out.

I suggest you to consult general surgeon. Soft tissues ultrasound and X-ray should be done. Hematoma drainage should be considered.

Hope this will help. Feel free to ask further questions.
Regards.

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