Thank you for using Justanswer for your question.
I believe you are taking Klonopin, the trade name for clonazepam.
You itching could be from either thing you mentioned. A way to discern which may be causing the reaction is to observe where your itching is. Is it only under area's where you have worn clothing? If so, that could indicate laundry detergent allergy. If the rash is every where, it could be from the Klonopin.
If the itching seems mild, I suggest you take an allergy medication like Zyrtec (recently became available over the counter, it is a non-drowsy formulation, so it should be taken during the day). You may take Benadryl or other brand of dipenhydramine at night (unless you have history of problems with either medication).
If at any time you begin to have difficulty breathing, your lips, mouth, or throat begin to swell, call 911 as these symptoms could indicate you are going in to anaphylactic shock.
I recommend you call your physician tomorrow to report this side effect and get recommendations. If your itching is bothersome, you can seek care at the ER today. They could give you an injection of an antihistamine or cortisone medication.
If you have questions, please reply.