Sounds like you set a value for feather or something along those lines. Select one of the selection tools, then see what options have a value in the top tool bar.
I set the feather to 0 and still the selected lines get distorted and not the same as the original selection
In the top tool bar for the pen tool, which shape is selected? I am thinking the rounded rectangle is selected.
If that's not it, then either post step by step what you are doing and/or post a screen shot so we can see what your entire screen looks like. It will get figured out.