Sudden side effects from birth control

So I’ve been on BC for about 4 years now on the same pill ( Levonorgestrel & Ethinyl Estradiol). I remember the first week on it, I experienced nausea but after that I had absolutely no side effects at all. Up until several weeks ago ! I’ve noticed that some days after I take my pill especially on an empty stomach (which isn’t new ) I am incredibly nauseous. On the week without my pill, I had a very normal period and felt fine. Now off my period I am experiencing occasional nausea again. Why could this be?