Help! Something bit me, somehow.?!

Hi, just a few minutes ago i was just sitting at my computer when i felt a little pain on my knee. I checked it out and it was two little bite marks about half an inch apart. I was wearing sweat pants and they were covering that area. Now its just a little swollen with that little bump and the bite marks abovt it, what do you think bit me? i checked around and nothing was in my room, the bite is like this . . please answer! Should i be worried? Thanks :)