TTC baby--- doing some research & came across this. Just for future reference.
QUOTE: "There are very few, if any, pregnancy symptoms before implantation, since the embryo has not begun to make hcg. However, your body does not continue *as normal*, or with business as usual. After hatching from the zona pellucida, the embryo sends a signal to the ovaries to secrete early pregnancy factor (EPF), which occurs from 1-2 days after fertilization. This EPF begins a dialogue between the embryo and the mother. It is possible, therefore, that a woman who is very sensitive, or is accutely aware of things, may have some symptoms associated with the EPF secretion, although I am not sure if this has been studied." End QUOTE
I just read that EPF is an immunosuppressant, which means that it lowers your immunity. It does this so your body doesn't attack the embryo and treat it as a foreign entity, before it has the chance to implant.