Thanks for posting your query
From description it seems to be muscle or ligament strain. If it getting worse day be day it needs thorough examination and mri
of knee to know the exat cause. So I suggest you to consult an orthopaedic Surgeon
for the same for definitive treatment. In interim do hot fomatation, apply locally anti inflammatory gel and rest well.
Hope this will helps you.
Wishing you good health
Dr Saurabh Gupta