Disclaimer: I'm not a doctor

A lot of women who have heavy periods are given the pill so they have lighter periods and less cramping and whatnot.

I have had the same experience as you - on bcp for several years; often no bleeding during that week where it should happen. My doctor has told me repeatedly that it's nothing to worry about and that the hormones are still doing their job.

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maybe she's born with it, maybe it's chlorine
If you're injured and need some sympathy, PM me and I'm very happy to write back.
disclaimer: PhD not MD

If it helps, my last 2 periods have been so light (pretty well just spotting) that I actually took a pregnancy test to make sure (since I've heard of women spotting while pregnant). It freaks me out! (and I'm not preggers, phew!!!!!!) I think it's length of time on the pill plus stress - have you had any extra stress/emotional changes in the past year?