I ate dark chocolate and then felt sick all day and finally threw up. could i have an allergy to dark chocolate?

Probably not: I assume that it is not the first time you ate it, didn't you? It could have been some indigestion or possibly food poisoning especially if nausea/vomiting were the only symptoms. Just let your stomach recover and try again but only a small bite a time to test it.

Less likely.: If you've tolerated milk chocolate well & tolerated dark chocolate previously, a new food allergy is less likely. Too much of anything can upset the stomach! hold off on anything chocolate for at least a month (you may not want to be near it anyway after it made you so sick), then try only a thumbnail-sized amount the first day. It's healthiest to stay with only small amounts of any treats, too.