That is a tough one, since most allergies present after multiple exposures without a reaction. One way to know for sure is once you are sure the hives are all gone, try taking another dose of ibuprofen. If you don't get hives, you are not allergic.
The problem with hives is that they are really widespread and can be caused by practically anything, including exposure to sunlight, sweating, or even emotional upset. People who have autoimmune disease get them, as do people who don't have it. I doubt there is any indication that this would be a marker for Crohn's. I don't have Crohn's and I get hives for no rhyme or reason. My son gets hives, and does not have UC, but he has fairly severe seasonal allergies.