I read carefully your query.
I think that you should do knee ultrasound
or knee MRI
and than you should go at orthopedist or rheumatologist
depend from your problem who may be:
- liquid in you knee from rupture of a ligament ,rupture of menisc or inflamatory disease
- Becker cyst(cyst synovial outside from normal capsule of knee.
Wishing you fast recovery.