I'm not on any pills, haven't been since before my kids were born. I know people though who have had great results with it and others who haven't. I think it is all based on individual body chemistry. If you decide to try them, give it about 3 months for your body to adapt and if you aren't happy have your doc switch you. Since you don't need it for conception, I would recommend asking your doctor about one of the ultra low dose or mini pills instead - their use as a contraceptive may be as low as 90 % but the side effects are much less. More info on some options here: http://www.buzzle.com/articles/low-dose-birth-control-pil...
One other thing, You don't say your age but since you have had 4 kids I assume you are at least in your 30's. Peri menopause can begin as early as your 30's and usually no later than your early 40's. Ways to know can include an increase in PMS symptoms, more mood swings and irritability, and irregular periods. From what you describe it might be a possibility for the change. You might read up on it and see if it matches your symptoms.
Also regardless of PMS or perimenopause there are some natural non-hormonal ways to help - Evening Primrose herbs are the best, but Flax Seed oil, Vitamin E, and Vitamin B6 can also help. So if you are unsure about the pill, you could look into natural options.
Good luck with whatever you choose!