for the past couple of months, I have started by period a week early. Not heavy but light and dark brown(when I wipe) and with some small period pains also.

A couple of months ago I went on holiday and forgot my pill, so missed it for 2 weeks. This screwed up my cycle and I went to the docs twice in early may - I had two pregnancy tests at the docs and these were both negative - anyone got any ideas as to why this keeps happening?

I have been on the pill for 8 years now and never had problems like this before.