I know how you feel, I started walking last week and I skipped Sunday and felt bad about it all day. It really is a good idea to let our bodies rest when they need to, and instead of feeling guilty plan to gently increase your activity until you are back at 100%. Park your car farther away, take the stairs etc. Just add a little bit of light activity while you are sick so you can still feel successful and in control.
And get yourself back into the "Losing for April Fools" thread, we need you there!
Sheri