Feel so shy and awkward

When I've been with a partner for a long time it's weird how shy and awkward I become about sex and romance. Really every form of intimacy, even basic emotional expression. It's so bizarre feeling quite comfortable with those things in the beginning, then feeling progressively awkward with time. It's not just lack of interest (at least earlier on), it feels like shyness, or like it's embarrassing. It feels like the opposite of most people's experience. Anyone else relate to this?