If you just updated to Photoshop CC 2018, you may have noticed a small purple line that follows your brush around, and until you figure what it is, you may be frustrated by it. Retoucher Pratik Naik put out a great video explaining exactly what this purple line is.
Turns out this purple line is an indication of a feature called smoothing that is turned on. Smoothing makes your brush strokes… well…. smoother.
There are quite a bit of options on that tool and the video does a great job of breaking them down.