Pain, Dark Spotting After Taking Ovymesen. Cause?

Hello. I switched BC pills from Cilest to Ovymesen (sic) a couple of months ago. I ve taken the pills as instructed, although not always at the same time each day. I started the third pack of Ovymesen after my 7 pill-free days about 17 days ago, but yesterday I started getting period pains and dark brown/light red spotting, even though my period isn t due to start for another week. Could you tell me the likely causes for this? Overall, I ve been on the pill for over 5 years and have only had breakthrough bleeding before after taking two packs back to back.