No she wasnt drunk when she wrote this song, it has a hidden meaning, she isn't talkin about umbrellas and rain, shes talking about love and hard times. Rain is a metaphor for the hard times, the bad times when everythings falling apart, and the umbrella is a metaphor for protection, shes saying shell protect him even when things get hard, shell be there for him and protect him just like an umbrella does when it rains, (even tho umbrellas don't work that well as they alwasys break, get caught in a tree or just get blown away by the wind!)