Late Period, Pink Spotting, Headache, Lower Back Pain, Clear Discharge. Implantation Bleeding?

My periods are never late. I had sex 5 days before my period was due and my husband left it in. I had mild cramping or burning sensations about 3 days late, at 6 days late i took a test and it was negative, I called to make a doctor appt when i was 9 days late she told me to wait til i was 14 days late and retest then call back. On the 12th day late I started spotting periodically throughout the day a very very light pink when i wiped. I put a panty liner in thinking i was getting ready to start but never did, that light pink spotting has lasted for 4 days and there was one day it was only twice when i wiped and it was a light brown color. No flow whatsoever. I am now 17 days late and going in tomorrow for blood pregnancy test . I have had headaches , lower back pain , and the first few weeks excessive discharge clear . Could I have experienced implantation bleeding? and what is the chances I am actually pregnant? Ive never done this before and I have a period every 3 weeks give or take one or two days at the most.