I Have Been On Cerazette Mini Pill For Years Which

I have been on Cerazette mini pill for years which I have more than happy with. These stopped my heavy painful periods completely. However, my Doctor prescribed me 6 months supply of Cerelle the middle of last year explaining that these work exactly in the same way as they are the same ingredients but a different brand name and are cheaper for the NHS. I have had a couple of months of very slight spot bleeding which just lasted a day but this month I have period pain and a brownish/black discharge lasting two days so far. Please advise.