Layer mask glitch?!

hey!

i am having a right nightmare with my photoshop at the moment. All of a sudden this problem popped out of no where and I’m not sure what to do to fix it!

basically when I create a layer and I do an action, say for arguments sake a Gaussian blur, I apply that to the layer and turn my layer mask to black to then paint In where I want it to blurred. But when I paint in , it’s fine but when I go over to apply more of a blur it erases what I just painted in? I want to increase the opacity by painting it in slowly but it just paints in and then on the second stroke it undoes what I just did!.

I hope ot can make sense to someone but I need this resolved quickly! . It works like that for everything so if I play an action and I’m doing skin smoothing or background fading just anything that requires me to paint in where I want that action to be visible, it just undoes it and erases what I just painted in and you can’t paint it back In!!

i really hope someone can help and is able to give me a resolution I’m tearing my hair out!