basic brush help

Considering I'm very new to photoshop this might be a very simple question and I have really overlooked on how to fix it.
The way that I cg is very simple, new layer+lasso tool for filling in areas+simple brush tool to fill in all areas missed. Only the last few times I've tried to fill in any missed area's my color is coming out darker everytime I go over it as if coloring with a marker. Before it stayed one color and i was able to fill in area's quickly and with the exact color that i've chosen. I'm not sure as to how to get it back the way it was and I have no idea what I have done to change it. Anyone that can help I would greatly appreciate it.

Make sure the brush opacity is at 100%. Check the options bar.