My 3 Year Old Had Red Cheeks, Decreasing Appetite, &

My 3 year old had red cheeks, decreasing appetite, & drooling starting approx 5 days ago. Assumed he was getting final molars, so treated as such....but he now seems more & more lethargic & ill seeming. Had low grade fever 3 days ago, responded to Motrin (which was primarily given to help sleep as he appeared to be in pain), has not needed since 2 days ago, but appetite and energy continue to decrease. Low fever still present, but wehaven't given Motrin since Saturday, as he doesn't seem to be in as much pain (or is he just too lethargic to be fussy?) No cough, runny nose, vomiting, and #2 is normal for him. Really only willing to eat/drink fluids and doing well with those as far as volumes (milk, juice, smoothies). Could lethargy be from prolonged calorie deficit, or are we missing something more serious?