Most Helpful Guy
There's no logic in that. Guys take a breakup differently than girls. While girls are often sobbing away immediately after a breakup, guys are usually socializing with his friends and exploring his new found singleness. Often, this can result in hookups shortly after a breakup.
I once had a breakup and within 24 hours, another girl that I had been friends with for years decided to "make me feel better" by jumping in the sack with me. If there's a girl that likes him during the period you were together, she may take the opportunity to steal him away for good by sleeping with him to ensure you don't get back together with him. Or he might just go find a girl himself.
Most Helpful Girl
Not at all, maybe just his way of moving on, lots of men do. Women seem to linger in our feelings and take time (not all girl) before we get involved, well more then 2 weeks anyway. But no just because he slept with someone else 2 weeks later doesn't mean he was cheating on you.