I was put on the pill for my painful periods. it regulated them and decreased the pain, but i started getting periods very week. what happened?

Breakthrough bleed: Sounds like breakthrough bleeding which occurs when you have bleeding at unscheduled times while on the pill. Brown is simply old blood that has been sitting at the back of the vagina. Often things get back to normal the next cycle. If not then schedule a visit to discuss changing brands or changing birth control methods. This is very common. Other than being annoying it is not dangerous.

See a doctor: Usually frequent intermenstrual bleeding while on bcp is due to low estrogen levels. This can be fixed by changing the pill you're on. There are other causes for this, though, like infection, or anatomic anomalies (e.g. Endometrial polyp).