Why am I so sad?

I've been with my boyfriend for just over 3 months and I could not be happier. He treats me like a princess, and is everything I could ask for and more.. We have sex regularly, everything is like a normal relationship, but I just get so sad and I start thinking things are wrong when they aren't.. He's always there for me, but I over analyze everything and cause problems that aren't even there.. Help? Advice? Anything please? Why do I keep feeling/doing this to us