I have been on the pill since I was sixteen, which changed my periods dramatically. Before that, they were extremely heavy, lasted about 10 days, and were about 35 days apart. However, I didn't have much cramping. As to mood swings, it's hard to say -- as a teenager, I pretty much was a mood swing, so I'm not sure how much of it was PMS.
Over the years I've been on the pill, my periods have gotten lighter and less painful. Occasionally, I don't menstruate at all. When I was running a lot and had very low body fat, I didn't menstruate for about six months.
As to PMS, that has definitely gotten worse as I've aged. It's hard to say how much worse it will get, and god help us all when I go through menopause! I had a Dr. prescribe me 10mg of Prozac a day for the 5 days leading up to my period, and it did help with the PMS, but it made me unable to orgasm, which sucked big time!
I get very horny as my period approaches, and being sexually frustrated (unable to come) made it not worth it to take the Prozac. So now, I pretty much tough it out -- try to get plenty of rest, and warn my Master about what time of the month it is, so he's prepared for what may seem to be unusual moodiness or irritability.
Lee, if you don't take the pill now, and you're not at extra risk for breast cancer, you may want to consider going on it. It will probably reduce your cramps. You could also ask your Dr. about the temporary Prozac -- it doesn't have the same sexual side effects for everyone, and if the moodiness is seriously interfering with your life, it may be worth it.
(NOTE: Prozac was briefly marketed as "Sarafem" for Premenstrual Dysphoric Disorder, or PMDD...but I don't think I have that disorder; I just get really bad PMS.)