What are employers allowed to ask former employers?

Hi I'm wondering if there are any rules in particular for the things a potential employer can discuss with a former employer when calling about job history, etc. The reason I ask is because I worked at an apartment complex a year back, I was there for a few years and everything was good. Well I got romantically involved with one of the girls in the office, things didn't work out and it got pretty ugly between us. The whole job was akward and she was trying to make my life at work a living hell to get revenge on me I guess, so I gave my two weeks notice on paper and quit. However, now when I apply for jobs at apartments(i've applied to 5 different complexes now), they don't seem to want to hire me despite my experience, professional appearance and positive attitude. So are there any rules in place to protect workers from slander like this? Thanks.