Should I be suspicious?

I broke up with my boyfriend back in June after 3 months of dating. It was a pretty ugly break-up, and though it took me a while, I did eventually get over it. About a week ago, though, I got an unexpected surprise when I found a note on my doorstep asking me to phone my ex. He told me that he was sorry about the way that he had treated me before, and that he had thought about me "off and on" since we broke up 5 months ago and that he wants to give it another try. Is it possible that he's being sincere? Is 5 months an excessively long time to go without trying to reconcile with someone you have just broken up with if you really miss that person? If he really had had feelings for me, wouldn't he have contacted me sooner?