Hello,
Thank you for your query to DoctorSpring.com
The symtoms that you have described do not appear to me to be an allergic reaction to taking Allegra. Having an allergic reaction would have a more dramatic response such as hives, swelling of your tongue/lips/face/other parts of the body, shortness of breath, abdominal pain etc. Allegra is an an over the counter anti-histamine used to treat allergies. Since it was a single dose, please don't worry about it, it will leave the body on its own when you urinate.
Your symptoms appear to be a continuation of the cold symptoms which requires bed rest, plenty of fluids and symptomatic management only and the symptoms will resolve with time. Having a fever (anything above 98.6 F) can be treated with acetaminophen 250 mg. take 1 tablet when you record the temperature with a thermometer as above 98.6F, you can take them as needed but make sure to not take 2 tablets less than 4 hours apart or more than 6 tablets in 24 hours.
Hope this helps, feel free to discuss further