Abnormal bleeding can be the result of several problems. Never use the first or last few years of your periods (puberty/perimenopause) as a sign. These will naturally cause these problems. If you aren't in this timeframe, there are several symptoms you would want to watch for before contacting your doctor. If you have periods longer than 7 days, or soak through a thick pad or tampon is less than 1 hour, you probably have menorrhagia (excessive menstrual bleeding). Clotting is not necessarily a sign of anything bad, unless there is any possibility you are pregnant. Bleeding between periods can also be ok - especially if you are ovulating or trying to concieve, because this may be a sign of implantation.
Definitions: amenorrhea means no periods at all, usually for a 3 month timeframe
dysmenorrhea are painful periods.
Ryan's Pharmacy has a pretty good list of things to go over regarding irregular bleeding. Ohio's ReproMed is good for hormonal disorders information.

Causes
Usually the normal upds and downs of hormones at different points in the cycle aren't working the way they should. Fibroids, polyps, IUD's, or any pelvic disease can cause abnormal bleeding.

Treatments
Lots of times, the pill can easily treat hormonal problems. If the problem is fibroids, it needs to be removed. A D&C procedure may be used to scrape out old linings of the uterus. There is also a new procedure called endometrial ablation that uses an 'iron roller' to burn out the uterine lining. Hysterectomy is the last resort for this type of problem.

Resource URL's
Dr. Speroff

Abnormal Uterine Bleeding
Abnormal Menstrual Bleeding
Heavy Menstrual Bleeding
Abnormal Bleeding - Women's Health Net Links
Abnormal bleeding
Menstrual Disorders - InfertilityPhysician.com
Irregular Menstruation Seen Raising Diabetes Risk
Menorrhagia and dysfunctional uterine bleeding
Dysmenorrhoea

asked me if I would put up some information regarding dysmenorrhea. The links you see here are her work and suggestions.

Dysmenorrhea are painful periods. There are a number of causes and treatments for this condition. Also see: Menstrual Cramps (Dysmenorrhea)
Projectlinks Database: Dysmenorrhea Hub
RXmed: Dysmenorrhea

Sometimes the uterus is tilted the wrong way, what is known as retroverted rather than anteverted. These 2 links should give you some ideas on that issue:
dyspareunia.org
dysmenorrhea.org

Rather than post this one link on the support page, I've added it here:
Dysmenorrhea Support

Last update: April 26, 2006