I doubt that it's the weed. It could have been caused by the cracker.

It may be: Dermatitis Herpetiformis (DH), which is an important associated disorder or complication of celiac disease which is manifested in the form of a skin rash.http://www.csaceliacs.org/dh_defined.php

The reason that I mentioned it is because you seem to have gotten this after the cracker, which probably contains the ingredient Gluten. People with celiac disease (spelled coeliac in England) can not ingest Gluten, which is a protein found in wheat, rye, and barley. I have Celiac disease, but didn't get Dermatitis Herpetiformis.

It could also be a wheat allergy - which is a different condition than Celiac disease. Wheat allergies can also case skin rashes. Wheat is now listed in the top 8 food allergens along with peanuts, tree nuts, milk, fish, shellfish, eggs and soy.
http://www.wheat-free.org/wheat-allergy.html