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If the knee is painful and very warm to touch then you need to see a doctor to have an evaluation in person. The first thing to rule out is septic arthritis which is an infection in the joint. this can be very painful but if not treated then can lead to destruction of the joint.
Other possible causes of a single swollen knee include gout and arthritis. I would suggest examination, some blood test and even aspiration of the joint if there is palpable swelling.
If it is recurrent that this may possibly be gout, in terms of the night-time onset, that may be due to the fact that leg is immobile during the night and allow gout crystals to settle in the joint.
The fact that it is warm is enough to suggest an underlying inflammatory process. The other signs of redness and swelling may occur with time but as a young healthy adult then your body defenses may be more effective at 'mopping up' a problem before it takes hold.
During an exacerbation of pain and heat is when you need to get assessed, so if the appointment is not in the next 24-48 hours then you may need to consider urgent care.
The actual cause is increased urate and this is related to purine metabolism. It is common in people with meat intake however it can be triggered by drugs and also some kidney problems. A sudden change in dairy may actually be the precipitant but we need to have the results of aspiration to be sure.