Gradient issue when painting

Hi Kent! 

I'm getting a weird result when texture painting. 

Basically, when I apply paint on my model (typically when the object is filled with the base color, and I'm about to do a first pass with a darker color), instead of a smooth color variation between the tones, I'm getting some weird circular gradient between the two colors. 

It's like if the gradient going from the darker color to the base color was really really obvious and made of distinct circles, each circle with a slightly different color. 

Does it have something to do with the stroke method ? I followed the tutorial pretty thoroughly and my settings seem to be right. 

Any idea when this could come from? If my explanation is not clear enough I can try to join a screenshot! 

Thanks for the help, and the awesome courses =)