Is it common to feel sick just after you have started a (diet and excercise) weight-loss program?

Applesauce McGee asked:


I just started a (much needed) weightloss program on Saturday and I felt fine then and on Sunday but now today (Monday) I basically feel like if I’m coming down with a flu. It it because my body is reacting to the change in activities and my new diet or am I just coming down with the flu.