I noticed that you have a 12 week old! Have you had any returning signs of fertility like previous bleeding or fertile cervical fluid? About 2/3 of women will ovulate prior to their first postpartum bleed, but only half of those will have a luteal phase long enough to sustain a pregnancy. Breastfeeding will affect your returning fertility as well. Do you chart or have any idea if you're ovulating and if so, how many days past ovulation are you? Implantation occurs between 6 and 12 DPO and 84% of the time between 8 and 10 DPO. But, implantation spotting is really not common, despite what you'll frequently read on forums like these. And spotting for so many days would, to me, not point to implantation either. However, spotting/light bleeding when pregnant is not uncommon around the time of expected AF. Some women get accurate results as early as 8 or 9 DPO if they had early implantation, but many need to wait until 12-14 DPO. What tests have you been using? Be sure to pick up one that only requires an hCG level of 25, has pink dye, and is not the +/- variety. First Response Early Response is a popular and reliable test, but dollar store and many generic drug store tests are just as good and cheaper.
Otherwise, I would say you're just experiencing an odd cycle caused by hormonal fluctuations. Breastfeeding can cause lots of fluctuation in your cycle and of course there are other common things can affect our cycles too, like travel, illness, injury, stress, diet changes, etc. As soon as you see blood and fertile CF, you should consider yourself potentially fertile and take precautions if you do not wish to be pregnant at this time.