This is due to the "Transfer" (called "Other Dynamics" in earlier versions of Photoshop) settings in the brush panel and having "Opacity Jitter" set to "Pen Pressure". Change "Opacity Jitter" to "Off" and it should behave how you mean it to again.
Since you haven't used it in six months here are the steps:
1. Select the brush tool
2. Go to Window > Brush using the menu at the top of the application window
3. Check and select "Transfer" (or "Other Dynamics" if you don't see "Transfer" there) on the left side of the new window
4. Change "Opacity Jitter" to "Off"
You probably want "Flow Jitter" to be set to "Pen Pressure" if it isn't already.
Remember to save this brush to save these settings (right-click on the canvas, click the top right arrow in the new window and select "New Brush Preset").
Now that I think of it, your problem could be as simple as you just need to choose a different brush ... have you tried selecting the brush tool, right-clicking on the canvas, and selecting a different brush? Try the ones there, one of them probably already has the settings you remember.
I'm pretty sure this will solve your problem but let us know if it doesn't.