You don't catch it from 'sleeping around', being sexual with one person who is infected can be enough... and this appears to be what's happened to you.
He needs to be tested too, but it is extremely likely that his test will be positive which very strongly suggests that he's had more than one partner (it doesn't need to be vaginal sex, oral sex will transmit this) and the only question is when.
Have you ever been tested for it before? If not, it's possible that you have had it for many years.