Fever but normal labs?? Advice!

Okay so my son will be 30 months in 3 days. We went out of state this past weekend to see my great grandmother. Saturday night, he was very irritable and would scream every time someone moved him. He does have constipation issues so we thought it was gas or he was really constipated. I knew nothing could be broken because I never let him out of my sight.. Anyways. Around 2 a.m. Sunday morning he starts screaming all of the sudden when he was sleeping. I picked him up and he was burning up. We rushed him to the hospital and his fever was 103.1 they rushed to get an IV in him, drew blood, got a urine sample, gave him motrin, and took and xray of his belly. By the time the labs came back his fever had broke and he was sleeping in my arms. The Dr said all of the labs were normal, even the xray, and the urine sample.. So he concluded that my son must just have a virus. He gave instructions and dosages for my son to alternate motrin and Tylenol every three hours. I followed the instructions and wyatt was fine, back to his normal happy baby self.. So last night (Monday night) I laid off and stopped the medicine since he's so young I did not want him to be on something constantly if he didn't need it.. Well, this morning (Tuesday) at 8 am he woke up crying and had a fever of 100.4... I gave him Tylenol and am waiting for it to kick in. I'm still out of state (we have been traveling for two days) but will be back later this afternoon and plan on taking him to the doctor but I need some advice. Has anyone experienced the same thing? Do viruses truly last this long? What could this be? I'm so worried about him. Anything would be helpful.. Thanks you guys.