Your depo shot is not likely to have any effect after so long. You may be experiencing some hormonal
disturbance as your chance of ovulation is less as you are on pills; even then if a failure is suspected, your ovulation date should fall around 8th-10th of January and if you have by chance conceived, you would not be having spotting so soon as implantation
bleeding is seen 10-14 days after conception. Getting consecutive negative pregnancy
tests almost rules out a pregnancy. Maintain a healthy body weight, get some regular exercise and eat healthy. See your doctor for further advice. Take care.