I went to my doctor after I was walking to work one morning and saw a bus coming and actually took a step to throw myself in front of it before I pulled myself together. Later that day I started bleeding and was literally like someone flipped a switch and I didn't feel suicidal anymore. Which made me feel like I was loosing my mind because who goes from 'I want to throw myself in front of a bus' to 'I'm perfectly fine' just like that? I did some research, I went to the doctor and described my feelings, he looked me in the eye and gently asked what I thought it was, I said I'd read about PMDD and I thought it might be that, he said 'I think so too' and wrote a prescription.
If, before you get your period, you feel furiously angry, suicidal, irritated by every tiny thing to the point you want to murder someone, stuck in a black hole you'll never escape from. If you are experiencing extreme emotions for what seems like no good reason, especially if you get your period and those extreme emotions just go away. You're probably not just PMSing , you may have PMS's feral big sister PMDD and it's treatable.
Also this is something that can develop as you get older. So if you used to get normal PMS but what I wrote above sounds more like your norm now then don't just write it off as regular PMS.