What Causes Delay In Periods And Nausea?

Hi I am on the pill and have not long had a period, I have been getting a bit of nausea and am currently taking sinus meds, I have 4 children now and have been told by docs I m very fertile....can I get a period on the pill and still be pregnant???? i did a home pre test and there is a very very very faint pos line...little confused