I started using Latisse about a week ago and noticed itching all over my body, is this an allergy?

This is unlikely due to your Latisse use. If it persists, I recommend following up with your Doctor.  Keep in mind, eventually Latisse results may plateau. A combination approach is needed. In combination with Latisse, I typically inject and apply topically PRP for a few treatments at the beginning to improve the growth rate of the lashes. Take viviscal as well. See an expert for a formal evaluation. Best, Dr. Emer

This is a good question but also a complex one which I can’t answer fully via this forum. Latisse contains bimatoprost as well as the preservative benzaalkonium chloride (BAC) at a concentration of 0.05 mg per mL. This preservative has been around since 1935. It is quite common in many products, especially those used around the eye. BAC is more likely to cause irritation than true allergy but certainly cases of allergic reactions can occur with BAC even serious ones. 
Studies at the Mayo clinic published in 2016 (see reference below) suggested that allergies to BAC might be increasing in the population. In fact, the allergen climbed up the list to now be one of their top 10 allergens they see in their clinic.
It is far more likely that an allergy exists if the eyelids show some redness. But even without redness a systemic allergic response is still possible. With these sorts of situations, I like to know the whole story (and see the rashes if they are present). I like to know if the patient uses products containing BAC in the past and if so, what types of reactions they had. It’s a good idea to review the whole story with your physician. If it was only itching, one might (on the advice of the doctor) wait for the itching to go away completely and either try again or have a specialized physician test the product’s allergic potential on the skin as a prick or patch test.  However, if there was any runny nose, cough, chest tightness, wheezing, shortness of breath, dizziness, throat tightness, swollen tongue or eyelid swelling...one should should seek the advice of an allergist physician.

Wentworth A et al. Benzalkonium Chloride: A Known Irritant and Novel Allergen. Dermatitis 2016.

Hey txrn69, Sorry to hear about your itch! Its unlikely to be a result of the latisse. Most of the latisse's side effects are localized to the eye and the periorbital area. These include eye redness or discomfort, eye itching, dry eyes/watery eyes, eyelid redness, or puffy eyelids. Hope this helps answer some of your concerns!
This answer has been solicited without seeing this patient and cannot be held as true medical advice, but only opinion.The information provided should not be relied upon as a substitute for consultations with a qualified health professional.