Click layer where your path is. In the top menu, go Layer>Layer style>stroke. Try different stroke "structure" settings until you find what you're looking for. You may have to minimize the pixel size to 1 if you have "outside" as the position.
I don't think he means stoking. This happens every now and then and I'd love to discover the button I accidently hit to cause it! Have a look at the top tool bar. Theres some boxes to the right which change the selectioin type, one of these might help.
I have managed to cure this in the dim and distant past but I can remember how and it hasn't happenend to me in ages. In the meantime just inverse your slection and you'll fill the right area. If I get a bump on the head and remember how to fix it properly I'll let you know!
use the path selection tool and right click, you have a "fill" option. The only thing I can think of that would make that happen is if you have a path already and your sub-path is subtractive (the little minus sign). That means you are removing a portion from the existing path. In that case, though, there should be a "fill subpath" option.