What are some signs that my boyfriend is in a rebound relationship?

my boyfriend and i were together for 3 months. we loved each other and he cared about me more then any other girl he's dated. I was the very first girl he kissed and i know he cared about me alot. a month ago i broke up with him b/c he lied to me and it just pissed me off. i learned my lesson and i will never do that again, but what i cant figure out is why he started dating someone else only 3 days after we broke up. if he cared about me so much then why does he want to be with this other girl? Is he on the rebound? if so how long do rebound relationships last and what can i do to get him back... if theres any chance at all that i can get him back after how i screwed up :/