I have problems whenever I eat oranges, too. That's what brought me here.
I avidly read your description because I always have problems whenever I eat oranges, tangerines, cuties or anything like that. Everyone always insists they know better than I do what's causing it, claiming it has to be the citrus, the sugars, etc. But I can eat lemon, pineapple, lime, and pretty much all other types of fruit, and had no reactions, but oranges get me every time.
Around middle school is when it started, at first I ignored it, thinking that I was just getting headaches from my glasses or something. As I entered high school I started getting stronger reactions. I LOVED oranges, and often ate them with lunch. Slowly the headaches got worse and I figured out that the oranges were causing them. I didn't finally swear off of them until I started getting stomach issues in addition to the headaches. I still had orange juice every now and again, but each time it caused the same symptoms; headache, stomach ache, even vomiting. After vomiting, my tummy would feel better but the headache would take a while to subside.
After three years of avoiding oranges, my boyfriend opened a tangerine, and I asked for a little slice, thinking I missed the taste and that maybe I'm over it.... I was dead wrong, my stomach immediately started hurting very badly and my head started pounding, I had to throw it up to stop the reaction.
So whatever you have, it's not an illness, no, but it's probably similar to me. Maybe it slowly took on, like my allergy, which is why you hadn't noticed it before