Opinions please

I switched from the IUD to the pill because of health reasons the middle of July. I had a regular period in July and August but now I am 6 days late. My husband and I did have intercourse on my "fertil" days and I haven't been the best about taking my birth control. I've been a little nauseous and lightheaded so I took a home test and it was negative...I guess what I need answered is if me taking my BC just every once in a whole and not regularly could throw off my period??