I think its not just the new physique that bothers people.....its the new you.. You look good, you feel good, confidence is rising. You are finding a new way of life. You are meeting new people in the gym. Your daily routine has changed. Its a lot for a partner to deal with if they have the slightest insecurity in themselves or in the relationship. You have to bring them along on the journey. Include them, and inspire them to feel confident in themselves and the relationship. Like any change in dynamics, it can cause imbalance.