Would you be happy for your partner to read or hear what you are saying?
Would you honestly be able to say that there isn't anything in the context of those messages/conversations that would upset your partner or break boundaries they've set? Are the context of the messages romantic or sexual in nature? Are you entertaining someone else's feelings or keeping them as a back-up? Do you prioritise not wanting to upset this person by distancing yourself/telling them no over how your partner feels about it? Are you trying to appear single to this person? Is this something that you're hiding or downplaying to your partner? If your partner was doing/saying the same thing to someone else, would you honestly be ok with it and not upset?