It can be due to patellofemoral pain syndrome as the symptoms are the peculiar factor of this....all you need to do is to start is take rest avoid climbing stairs for some time take painkiller to reduce pain and inflammation around that area you can take hot packs also to reduce pain and to increase the blood circulation around that area which will help you to reduce to reduce tenderness around that area
Also you can start static quads exercises with 10 reps and 10 hold , hamstring stretch, vmo strengthening exercises on bolster. Do these exercises twice a day. If necessary to walk or climb stairs use knee brace it will help you to transfer some load off from the knee
Next Steps
hot packs
painkilllers
rest
static quads and vmo strengthening exercises on bolster
wear knee brace while walking
always keep unaffected foot up while climbing up stairs whenever required but avoid as much as you can for now
Health Tips
painkilllers
hotpacks
rest avoid climbing stairs and walking for some time until the pain subsides
knee brace if required to walk or climb stairs
avoid using indian toilets
avoid doing squatting and sitting down on the floor try to sit on the chair
avoid cross leg sitting