My lines keep randomly lowering in opacity

Ok, so I am using Photoshop Elements 13. Whenever I draw, my lines keep lowering in opacity. I checked all my settings, all of them were how I usually have them: no pen pressure settings, normal. I was testing out different brushes, since the only one I use is the soft oil brush, and when I was testing them out it kept happening. And when they lowered in opacity, it kept drawing over the lines as if it was a clipped layer. I didn't even have any other layers! If you can help me, I will really appreciate it!