Let me stop you after word two of your question, because you've already deviated from reality.
Why would you think that? Just because the water was warm. If you start to feel weird in like 2 weeks then yeah you got a brain eater.
There are no brain eating amoeba in your head. Probably.
So yesterday I went with 2 of my friends to swim and a lake. The water was warm. I do not have any symptoms. I just wanna know if I have it or not. Stuff like this freaks me out.