I would recommend a ultrasound and tapping the contents, that should diagnose the condition. Considering the peracute appearance of the lump, I would consider a seroma, hematoma (aneurysm?), or synovial outpouching (weakness or tear) of the carpal sheath of the flexor tendons but there are a number of structures underlying and bordering that location and none often present that way when diseased.
Until diagnosed I would be cautious about exercise and consider wraps to reduce or at least support the tissue.
Some further questions that may help:
When you press on it does it bounce back immediately or form a depression that fills back slowly? And while doing this does the swelling then move to the other side of the leg?
Can you move the lump around as though it is not attached to underlying tissues?
Is it warm or the same temperature as the surrounding skin?
DrO