My period has been off the last two months and I'm wondering what's going on?

In June my period was 5 days late, and I took pregnancy tests but they were negative. Now its July and my period was 10 days late. Is something going on that I don't know about. I haven't been under any stress lately it can't be that. Also during the month of July when my period was supposed to show up I've been having most of the symptoms of pregnancy but the tests turn out negative. My period is always regular may 1 or 2 days late so I'd appreciate any advice that you could give me.