yeah, you really should check with your doctor tbh. loads of people's periods are all over the place (in a metaphorical sense ;-) ) so it can't be that uncommon to not be able to say 'right, today is the day four days before my period starts' sounds like the drugs company havn't actually spoken to any real women when they wrote the instructions :-)
don't worry yourself, just give your doctor a call, and they'll prob something like ^^ but it's best to check. xx