The spotting you experienced could be a breakthrough bleed that sometimes happens around the time of ovulation; since you have 31-day cycles, your ovulation would fall around day 16. You may go for the serum beta-hCG
assay one week prior to your next cycle; this would give you an earlier and a better confirmation than the urine pregnancy
tests performed 3-7 days after a missed period. Implantation
bleed usually occurs 10 days after conception. Please see your doctor for further assessment. Take care.