I work in a mental health center and have a coworker that recently was telling everyone that when she was 14 her mother had brought her in to the center and that she had a diagnosis. My coworker was laughing about the situation. I told her she could look it up to find out what it was. I later did so and told her as well as those she was joking about it with. Because we are all employees and have access to all of the records is this a violation?

I would have to, under the circumstances, yes it was a serious violation of the HIPAA rules. You should have stayed out of it, it was none of your business and certainly not to inform everyone involved what your coworker's diagnosis was. It may have seemed like a joke at the time, but this could have serious repercussions for you if your coworker starts thinking about and reports you. But the damage is done now. Never use the institutions personal records for a laugh.

I agree wholeheartedly with tickle. In my humble opinion, you violated one of the basic regulations that brought about HIPAA in the first place. I would be very surprised if your company's policy doesn't specifically state that accessing files for which you have no assigned task, is a clear violation. You most likely have access to all records, but only have authority to access records when it required in conjunction with your officially assigned duties.
The best course for you is for you to read your company's policy and the basic requirements of HIPAA, or find another line of work.

It isn't just a violation, it's unethical at every level, including your personal relationship with her. I hope you take some time to have a serious talk with yourself, because it should have been intuitive that it was wrong, regardless of HIPAA or who was laughing.

This is a blatant violation of the HIPAA laws. It is quite possible that you could lose your job over this if there is a paper trail on the computer program you are using, and I'm certain there is.