What's Wrong With My Little Man?

This weekend has been stressful. Davis has been sick off and on since last week and I don't know what's wrong. Last Wednesday night, he woke up throwing up. He only threw up once and then went back to sleep shortly afterwards. I kept him home from daycare on Thursday to make sure he was ok. He acted fine all day, except that he didn't eat as much as he normally does and had one super messy diaper/pullup in the morning. Since he was fine for the rest of the day and night, I sent him to school on Friday. He was fine in school on Friday, except that he again didn't eat as much an usual. He was active Friday afternoon and evening after school, acting like his usual self. Then Friday night he woke up around midnight again throwing up. This time, he threw up multiple times but eventually went back to sleep. Saturday came and again he was fine except for eating a bit less than usual and two super messy diapers/pullups in the morning. Oh, and when I got him dressed I noticed a small rash on his belly. Today, he was fine all day. Seemed to be almost eating like he normally does, was active all day, etc. Then about an hour ago, I came into the room to check on him and found that he had thrown up and gone back to sleep. I had my cousin, Alec come pick him up and move him to the couch so I could change the sheets. Davis barely woke up during that process and is now back in bed sleeping soundly.

What the heck is wrong with my kid?!? I'm so confused. He's not showing any other signs of illness, no fever or anything.

I had thought, maybe it was an allergic reaction to something but I can't figure out what it would be to.

I don't know. I'll be keeping him home from school and calling the doctor tomorrow. Hopefully, we can find out quickly what's wrong.