While it's too early for hCG to be causing the usual pregnancy symptoms this early, there is a hormone called EPF (early pregnancy factor) that is released within hours of fertilisation. You don't feel this hormone's effects, but it does compromise your immune system (so that your body won't reject the fertilised egg), which then can make women become ill or feel ill during the TWW. It's quite common for women to get colds, sore throats or even flu during the TWW and then get a BFP. When I was pregnant with my daughter I caught hand, foot and mouth from my son during my TWW and had a really high fever on 5dpo. It's quite rare to catch HFM as an adult, so I think I caught it because my immune system was weakened by being pregnant (just too early to know it yet).