Allergic reactions just happen. One day your body may tolerate a certain medication or food just fine ... and then another day you have swelling, itching, hives, swelling and trouble breathing! Yes, it's that quick sometimes. Your reaction time must be quick as well. As caregiver said, you need to carry either a 'epi-pen' or whatever medication(s) you and your Dr agree would be best in case of an allergic episode! Best of luck - take care!