It is always a bad sign if your partner seems to not be present at the moment, as there are several different explanations for this behavior - and all of them are negative. For one, if your partner is thinking about another person, they are not going to be really interested in holding a conversation with you. You should talk to your partner about this issue if it persists during a longer period of time.
Such a state of mind may scare the other partner into thinking that the relationship is nearing its end. While this may or may not be true, it is important to start working on solving the problem as soon as possible. The longer you let your partner linger in a state of limbo, the higher the possibility that something wrong is going to happen to your relationship, possibly ruining it forever after.