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Hello and welcome. I'm Dr Chip and sorry for the delay here
Is this pain in your calf and is there any bruising?
Then it isn't a deep venous thrombosis in a leg vein.
This sounds like a ruptured Baker's cyst
These can rupture suddenly without any major trauma to the knee.
For now, ice, elevation and ibuprofen will help and the pain may resolve in another day or two but you should have your doctor check it out and get an MRI scan of the knee.
Please let me know if you need any more information