Period came late, negative test?

I'm 25 and not using birth control my husband and I are not necessarily trying to become pregnant but if it happens it happens. My period usually comes like clockwork and my cycle is ALWAYS 28 days (never fails!) My period came 2 days late. I took a pregnancy test and it came back negative. I've been taking antibiotics since yesterday (amoxicillin) because of some tooth work i'm gonna get done. Would that have anything to do with a late period or negative result? Could I still be pregnant, any ideas?