Do you work nights? I know this is common for night shift nurses, and happens to me, too
. I have noticed that as I changed specialties, I am more tired than I was before. My job in the ICU is just more demanding physically, emotionally, mentally, so I'm sure that has something to do with it, but it is also that much more rewarding. Also, I think the longer you work nights, the worse it gets sleep-wise (for the people who aren't "natural" night shifters, which most people probably fit into this category).
Just do what you have to do, rest, but make sure you make sleep a priority in between shifts, as well.
What you're describing is why some nurses prefer 8 hour shifts to 12's.