Take an antihistamine, eg histal, benadryl, DPH and keep taking it as directed till the reaction goes away. Try to figure out what you are allergic to. Have you eaten any unusual foods (unusual for you that is), are you using a new soap, detergent, perfume, cleaning agent, food ingredient? If you have changed your routine at all, stop the new things, that may be what has caused your allergy.
Sometimes you can't figure out what it is, it may be something in your environment that is beyond your control - a plant, flower, tree, something at work.
If your tongue starts to swell, if you have difficulty breathing, it is a medical emergency, please rush to the hospital.
If you are not getting any relief from the antihistamines, please see your doctor, there are other prescription meds that may help.