Thanks for posting your query.
Your periods tend to get lighter when you are on the pill. In fact, the pill is often given to women with heavy periods
to reduce the amount of blood flow, as a treatment.
The fact that you are passing a lot of urine and have an ache on one side(of your tummy, I presume) may point to the fact that you may have a urine infection. If you also have burning sensation, when passing urine, see your doctor, who will test your urine, and if you have an infection, give you antibiotics.