PMS - is it just a normal part of life or a condition?

I'd say a bit of both.

As you say, in your case it's affecting your mental health - so that would make it more of a condition. I don't like this use of "normal" in this situation because every girl will know what's normal to them.

I'd say it gets serious if it begins to affect the people around the female or if becomes life threatening to her.

In general I think PMS is just something that may come or may not to every woman - but I'm almost certain that every female suffers from it at least once in their life!