Have I experienced implantation bleeding??

I'm 30 yrs. old. have 2 children. Never had a problem getting pg. never been on birth control. last period was aug. 20. started spotting on sept. 12 pink and sometimes light red lasted for 2 days only when I wiped. stopped for 24 hours and started again on the 15th and lasted only 6 hours. not heavy enough to have to put anything on. My period was due today however I always start early. I took a test on the 17th (today) and was negative. I still feel like I could be pregnant. Could that have been implantation bleeding? My periods are always very heavy in the beginning, I could never go without a tampon. I just know the bleeding i experienced was not normal for me. Is is too eary for a test to show positive? Could I be pregnant? thanks!!