I suspect my site is being hacked!

I change passwords constantly. My db, cp, ftp and admin. Someone is getting in and deleting pages and deleting my categories. I constantly try to back up from previous dates but the changes they make, still exist on the old web files from as far as 4 months ago. I have the backup files stored on my local drive as well as hard disk. Ive tried everything i can think of, what would be the next step i take?
BTW, i hired a developer about a month ago who had access but never started work and never heard from him again after hiring. After a week i cut off access. Possibly he installed some kind of virus. idk.. thanks

Also, i was getting strange emails from my site with links inside of them.

It wasn't what i suspected. As it turns out, they are getting in through the database and only the original developer had access. I changed the password but it doesn't help.

Are you sure you are not violating any rule with your site ? i think it is the people in charge of your hosting plan that is responsible for this.

Its a mysql hack. Here are some posts about it;

http://www.webmasterworld.com/webmaster/4006141.htm
http://www.webmasterworld.com/webmaster/3980743.htm
http://www.webmasterworld.com/webmaster/3880549.htm

Yes, its sql injection. Its hard enough starting out this business but for someone to deliberately try to sabotage the site is extremely frustrating. They are having a field day changing things around daily. Maybe they are practicing on me. not too sure the reason. As long as i have backup, they shouldn't be able to eff things up on me right? i have been constantly doing backups daily because of this and it always backs up correctly.

There shouldn't be any violations that im aware of. I'm selling plumbing fixtures and all is legit.