Lines become lighter when overlapped

Lately, whenever I use my brush and two lines on the same layer overlap, the part that overlaps becomes substantially lighter. I've attached an image below. I'm using Photoshop CC 2017 and the default brush, and I already checked that my opacity/flow/etc are all at normal levels. It just recently started happening, so I'm not sure what happened. It makes it almost impossible to ink or color, so any help would be greatly appreciated.