Welcome to Healthcaremagic,
From your Query, i suspect that you may have either backers's cyst or deep vein thrombosis
Baker cyst is not an uncommon cyst.
It is common in a patients with arthritis
or tear in cartilage
In both these condition, excess joint fluid is produced and bulges into the back of the knee.
Sometimes, this can protrude down to leg between muscles of calf.
This can rupture and cause bruise
and swelling in the leg.
Pain may be due to arthritis.
If it is a baker cyst, just aspiration and pain killer is enough.
Bruise with swelling can be due to Deep vein thrombosis.
To diffrentiate these, you have to do some investigations
2. MRI knee
3. Doppler study of lower limbs.
Hope this helps.
If any doubt, please reply.