To be euphoric is to be carried away with amazingly good feelings. The euphoric feeling of having won the lottery ended quickly when you saw your pet ferret chewing up your lottery ticket.
When someone's euphoric, they're so happy that they might have lost touch with reality for the moment. This “too good to be true” quality is one reason some drug-induced states get described as euphoric. In fact, euphoria, which euphoric stems from, originally meant a feeling of wellness caused in the sick by the use of drugs.