Your periods have been regular and you have been charting them well. If you have been trying to conceive lately and had sex during your fertile period , then the spotting on Dec 2 can might as well be an implantation bleed.
This can be confirmed by performing a serum beta hcg levels test between day 22-26 o f the cycle. If the levels are high, then you can be sure that you have conceived.If negative then there is no pregnancy and that bleed on Dec 2 was possibly a break through bleed which can happen in mid-cycle due to progesterone deficiency , could be secondary to stressors .
I hope i have answered your query in detail,
wishing you good health,
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