I'm going through a similar situation right now. Not sure if I am "on the rebound," but I do feel that the recent break-up with my husband / former primary is having emotional spillover in my relationship with my boyfriend. My relationship with my boyfriend has only been going on a few months now, while my relationship with my husband was 6+ years long. To be fair to my boyfriend, I feel that I need to create some distance in our relationship for me to grieve the break-up of the relationship with my husband. I don't want to break up with my boyfriend, but I do feel it's better we spend some time apart. I actually posted yesterday in Poly Relationships Corner looking for advice on this, and the advice I received seemed to confirm my suspicion that I should ask my boyfriend if it's okay to "cool our jets" for a while.
That said, I think everyone is different. Some people are probably more in need of alone time than others, and I'm sure it depends on the nature of the break-up, too: i.e., if it was expected, mutually agreed upon, sudden, etc.