Imagine that something happened in your relationship - maybe you had a problem or an argument you couldn't overcome. After that problem, the communication with your partner went lukewarm and you don't feel as close as you used to be. Try talking things through and maybe you could find out why your partner acts the way she or he does. If they refuse to do so (or they just feel distant and uninterested) then begin to suspect the worst.
This might be one of those signs that indicate that your partner is intending to or is already cheating on you. A lot of couples have said that before they caught their partner cheating there was an emotional distance between them. If this is happening then consider your relationship soon to be over. Just wait a bit to see what happens but don't wait too long because that is not in your best interest.