I don't think there's anything wrong with you, but many people's "mood" for sex is very dependent upon certain circumstances, like stress level. You may be to preoccupied upstairs to get into it downstairs if you get my drift.
Perhaps the best thing to do is to elaborate a bit with your partner on some of your fantasies, and tell him what really works for you, the same thing over and over, while it may be loads of fun, can quickly get boring. What you need to do is try and spice things up, talk with your fiance, see if you can try different ways of doing things, be open to experimentation, but talk about it first and see each other's limits.
Variety is the spice of life, to keep your sex life active and healthy you need to sometimes switch it up and try different things.