What Can Cause Visible Mucus In The Stool?

My almost 5 mo old son has had mucus in his stool & more frequent bowel movements all weekend. Today I picked him up from daycare with a fever of 101 & he just threw up on me....what do you think is causing these symptoms? I just realized I bought formula for 6-12 mo instead of the birth to 12 mo, but it is the same brand & type. Could that cause this?