First, have a discussion with your significant other. The question to explore would be does your significant other agree with you about your observation. If you and your significant other agree on this, it may be easier to address strategies in addressing this issue. Even if he/she does not agree, your significant other should support you being uncomfortable with this person.

The next question to explore is what role does this person have in your significant other's life? If this is a coworker or someone else that may be hard to avoid this person. Determining the level (quantity) of involvement between the two people would be a productive conversation to have. If this person cannot be removed from interacting with, the next discussion to have is quality of interactions. This would involve behaviors your significant may be doing that could encourage these feeling.

Lastly, evaluate yourself and this relationship. If this a theme between you two, the problems may be more significant. Or if this is something that has come up for you in other relationships, that would require some self evaluation regarding your own insecurities or choices in relationships.