It can become somebody's business is/when it impacts their life or relationships. OP feels some concern that her relationship has impacted her friendship and her absence has unintentionally hurt her friend's feeings.
I would agree if
the person in question were a close relative (spouse, parent, child, sibling) of the OP, and it really did impact them on some personal level. Otherwise, no. I just don't think that you have to bare your soul to friends and acquaintances.