This is likely to be Chronic Urticaria.
This occurs due to an allergic reaction to something that you ate or something that you are in contact with.
In your case, the contact with the weeds, most likely its pollen, is what set it off. You may be having continued exposure to the weeds or you may be having an allergic reaction to something else also. You should check out whether you started any new medicines or food supplements during this period that yo are having the attacks. Also see whether you have been taking any new types of foods like cheese, some types of meat, nuts, etc. It could be due to some new detergents or cosmetics that you are using, or some new type of soap,bath soaps, perfumes, etc. Link
If you do find anything which could be the cause, you should avoid this. Also, you have to be careful to avoid contact with the weeds. Continued contact with the allergen is possibly causing the welts to remain longer. Quite as often, however, the exact allergen cannot be detected, you should not worry if this is so.
The primary treatment of such urticaria is with an antihistaminic like Benadryl cap (OTC). You should take one capsule daily (it can cause drowsiness). If this does not help, then you need to consult your family physican or a dermatologist preferably. You should also apply Hydrocortisone ointment (1%) to the sites of the lesions..
Please discuss these treatment options with your doctor, I am sure your itching can be controlled with these steps.
You should read up here: http://dermnetnz.org/reactions/urticaria.html http://www.emedicine.com/emerg/topic628.htm
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