I Have Been Spotting A Week Before My Scheduled Period

I have been spotting a week before my scheduled period for the last 2 months. The first month, I took a Zpack for a cold I had the same week I was scheduled to have a period but it never showed; is that because of the antibiotic? (I took 2 pregnancy tests and they both were negative). Now I am scheduled to start my period this week however I had heavy spotting last week...I feel like my whole system is off by a week and I'm scared and don't know what to do.