Any ideas what could be wrong with my knee?

I woke up this morning with stiffness and mild pain on my left knee. I took a hot shower and got on with my day as usual. Now, 2 hours after waking, there is more pain and I noticed swelling around the meniscus. It is not a sharp pain but more of an annoying ache.

I am a 25 year old female. I used to be very active until I stopped going to the gym 2 months ago. I used to run on the treadmill for 30 minutes 3x per week. I am fairly sedentary now, but I don't sit still for long because I'm a restless person.

I did not hit my knee against anything and I did not kneel yesterday. I did paint a wall using a small ladder, but it didn't take much time. I put an elastic tube bandage on my knee now (tubigrip).

Any idea what could be causing this pain when I haven't done anything strenuous?

P.S: I had Osgood Schlatters disease between the age of 10 to 12 because I was a sprinter.

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