What Causes Mushy Feeling Knees After Running On Treadmill?

I was running on the treadmill for 30 minutes 10/km Hour speed no warming up. Im 19, slighly overweight BMI 26 and usually sit down all days infront of the computer but have now started exersicing.

After running those 30 minutes my knee feels mushy inside especially when im supposed to sit down on my knee (during some exercisines) and hurts alittle but only when sitting down. It feels like a mushy ball inside that get squeezed when I sit. I feels uncomftarbale to walk down the stairs. My knee has been like this for 2 days now :( but I have continued exercicing but only indoor cycling. Is it okay to continue exercising? Should I buy a kne cap? SHould I visit a doctor or will this go away by itself... please answer quickly!!!