nausea

my period is exactly 7 days away and I am on BC but last night and this morning I feel very naueasous. Im sure I'm not pregnant. recently, for about a week or so, I've taken my pill 3 hours late every night (but I haven't been having sex so I know I'm not pregnant). is this time change the cause of my nausea? Or maybe because my period is due in a week? I've experienced morning sickness before (while on the pill and no pregnancy) so I'm wondering if the pill is causing it. But I've been on it for about 4 months. someone please help