What Causes Irregular Brown Spotting?

I am 43 years old and obese. My husband and I do not use contraception and we had our first miscarriage two years ago. My period has moved up from the 4th of the month to the 12th of the month two months in a row. I now didn't get anything until the 20th and it is just brown and spotting. I have had it for three days now. No cramps except the 1st day. No other symptoms. Took a pregnancy test on the 16th of July and it came up negative. Can this be implantation bleeding or just an irregular period. Never had brown spotting on day one before.