Something up with my knee and I'm a little freaked out...

I know this has nothing to do with relationships or sex or pregnancy but I don't know anyone else to ask. Since yesterday my knee has felt tight almost like there is something causing resistance (if that makes sense) under my knee cap. I'm in the tub and just notice this. It mainly does it when I either get on my knees or have my legs straight. It's my left knee in case you couldn't tell. I'm a little freaked out so any feedback would be nice.