I discovered a small lump behind my knee pit towards the left of my left leg back in June. I was worried about it for a bit then completely forgot about it until a few nights ago. It doesn't hurt and it's movable. It's deep and you can't see it looking at my leg. I can only feel it if I'm standing up or flexing my leg so I can't feel it at all if I'm bending my leg. I don't think it's a bakers cyst as it's not big enough to be that. I also don't believe it's anything sinister but I'm a huge worrier. Has anyone else had this problem before? 
 

Thanks!