Sick angelfish

For the last 3 days, my angel fish has been laying upside down in the bottom of the tank.  The first time I saw it, I thought it was dead and went to scoop it out and it took off swimming.  The next morning I expected to find it floating but it was swimming around and came up to eat.  It has done this for the past 3 days.  Is this something they do or is it sick and probably checking out on me?