Trace back your activity for the last 3 days. Remember what you handled or brushed against. were you outdoors? touching plants or foreign objects. I suspect you picked up something that you are allergic to. like poison ivy for example.
You may have something called contact dermatitis which is caused by touching poison oak or poison ivy. You should see a doctor in case it's more serious than you think.
Could be an allergic reaction to something, such as new soap or detergent. Possibly, you got a chemical on your hands and is now a chemical burn. May be a dermatological condition such as psoriasis.