I'm in my early 30's and can remember a time when my period was light and I had not pain ... but it changed sometime a few years back and became VERY painful for a day.
I'm not sure if your symptoms point toward a disorder, or not. But you might be wise to visit your OB GYN to seek advice about the changes (color of the blood and severe pain). Can you describe the pain a little better? (Location, possible source, duration, etc.)
Right. As I said, if you suspect something is wrong, go see the gynecologist. But it sounds like you're describing the feeling of hemorrhaging from the inside walls of the uterus. I normally get that low, dull ache for at least 3-5 hours now when I never had it before. Seems pretty normal for a menstrual period.
Wow. That sounds quite debilitating. I suggest that you follow up by asking your GYN what's going on and what actions you can take in the short term (pain meds) as well as the long term (treatments) to help you during your period. Good luck and let us know what your gynecologist says!