Did you end up actually having a full out period last month? Or was it just the spotting on the one day?
Either way, a $1 test is worth it to put your mind at ease, IMO.
I had dark brown spotting started 5-6 days before my period was due and that turned out to be implantation bleeding. I also continued to spot up until about 2 days ago. With how out of whack your cycle seems to be right now, a test is probably the best indicator.

Good luck.