1. If the impurity changes one of the characteristics of the water (colour, taste or smell), this water becomes impure according to scholarly consensus, regardless of whether the water was of a great or small amount. 2. If something impure falls into a large amount of water, but it does not change any of its characteristics, be it taste, colour or smell, this water is pure, according to scholarly consensus.

The slight trace that results from sweat does not matter, and your prayer is valid. All you need to do is make sure that you clean yourself properly after relieving yourself, then do not worry about it anymore than you should after that, lest you fall prey to Satanic whispers.