Hi, I Had My Periods On 18th Aug And Had

Hi,
I had my periods on 18th Aug and had sex on the 7th day I.e. on 24th of Aug. I took Postpone 72 on 25th night. I had protected sex on 26th. On 31st Aug I spotted black fluid flowing from my vagina which eventually stopped within few hours. Then in the same night, I started bleeding just as if i am in my usual periods but 15days earlier. Because my expected period date was 15th of September. What does this mean - abnormal bleeding during to hormonal imbalance or side effect of the pill or is it my usual periods but at an early date? Does this mean that I am not pregnant? Should I expect my periods on 15th September again? I am really worried. Please reply soon.
Thank you in advance.
Bye.