Feeling Bad on the rare occasions when I eat junk food

I'm not a health nut, but I'd say I eat fairly healthy (rarely eat fried food, don't like a ton of cheese or heavy cream in food and try to eat plenty of fruit and veggies). But on the rare occasion that I eat fast food for a day (like 2 out of 3 meals have fast food, this is usaully on a road trip) I get a slight but annoying headache and my stomach feels kind of greasy for a few hours. Like I said, definetly not a fast food regular, but sometimes I have it as an occasional treat, but end up feeling a little bad. Is this normal?