Hey Ashish!
Intraarticular collection of fluid or joint effusion could result from infections, inflammatory conditions like arthritis, trauma, cysts, a few blood, systemic or metabolic disorders. If it is a single joint involved and in the absence of trauma or setting of an infection, inflammation is a more likely cause which could be primary or secondary to causes like degenerations etc. It would be best to discuss the likely possibilities in detail with your orthopedician/ rheumatologist for an accurate diagnosis, depending on which a management plan can be drafted accordingly.
Hope this helps.
Take care!