It is certainly possible that you are having an allergic reaction to the medication you took. It is also, however, possible that you are having a rash as part of whatever illness
you currently have, as many common viral illnesses may produce itchy hives and other types of skin rashes
. I think the only way to sort this out will be to go see your primary care doctor
, who can figure out what kind of rash you have.
In the mean time, you can take an anti histamine medication, such as over the counter diphenhydramine (Benadryl), which should help relieve the itch and let you get to sleep.
While you are waiting to see your doctor, you need to be on the look out for a serious allergic reaction. If you notice any swelling of the face, of the lips, or of the tongue this could be a serious life threatening allergic reaction and you would need to go to the emergency room for immediate evaluation. Similarly, if you have an wheezing, any shortness of breath, or any trouble breathing, or if you have any nausea or vomiting, you need to seek immediate medical evaluation.
Even if you don't have these serious symptoms, please do see your doctor as soon as is convenient.