You see your partner creating a special bond with others and not sharing with you about it. You feel confused because you know your partner loves you and at the same time you notice a sense of mistrust coming up in you. I have heard some refer to this situation as "emotional cheating".
The term "emotional cheating" is directed at the partner who making special connection with others and not sharing about that in the relationship, but what it really refers to is unmet needs on the part of the the person making the accusation.
If you are the one thinking that your partner is cheating on you emotionally, then you likely feel lonely, scared, confused, and have needs for inclusion, intimacy and clarity. What you value most about romantic partnership is the fullness of connection it can provide. One way of creating that fullness of connection is sharing all dimensions of your experience with your partner, e.g., play, work, health, transformation, spirituality, problems, celebrations, daily tasks, etc. A breadth and depth of sharing creates a sense of truly being together on life's path.
When your partner has close connection with friends and doesn't share something about that connection with you, you are missing out on an important part of your partner's life. Unfortunately when you bring this up with your partner, you might be bringing some reactivity with you, such that your request for inclusion sounds like an accusation. If your partner hears an accusation, then he or she is likely to withdraw more in order to avoid future conflicts. This in turn is triggering for you which makes it even more difficult to broach the subject in the future. Thus a reactive cycle has begun.
With any stressful and confusing situation, I encourage you to lead with curiosity. Your partner has some reason for not sharing with you. There is some need she or he is trying to protect. If you can be gently curious about your partner's process this helps to reveal needs on both sides. You can also begin these conversations with reassurance that you do not want to meet your needs at the cost of your partner's needs. When your partner truly gets that you are not a threat, he or she can relax and share more fully.
Reassurance is highly underrated. Most couples I work with are surprised by how much reassurance each person needs and how much it helps the relationship to move forward. There is no relationship in which you are more vulnerable so of course it is easy to slip into a reactive sense of threat. Accepting this as a given, you are able to offer and receive reassurance more regularly. It's especially important to offer reassurance when it looks like your partner doesn't need any. The need for reassurance is often covered over by anger, withdrawl, hyper-competence and self-reliance.
Reassurance isn't just about words. It's also about a gentleness in your voice tone, a warm embrace, a smile, a relaxed posture, a leaning forward in support, etc. Most important, ask your partner what helps him or her to relax and experience you as someone who is looking out for their well-being.
This week experiment with offering reassurance to your partner as least once a day in as many different ways as you think of. Notice what happens with your sense of closeness and sharing with each other.