1. a knee sprain
can result in tearing tiny blood vessels and bruising, both in the back of the knee and down the front.
2. are you able to bear weight on ankle or not? any associated numbness/pain?
3. Consider getting assessed by an Orthopedic surgeon for Examination, if the purple area continues to spread, who might ask for Color Doppler to rule out DVT.
PS: generally there isn't any association between pin worm and pinkish lump.