I had light spotting and crampiness and low energy. The crampiness was hard to figure out what it was - it sorta felt like I had to run to the bathroom but also sorta like menstrual cramps. It felt sort of in between the two. I was playing soccer when I had the most cramps and I remember complaining to my hubby (he was on my soccer team and playing right next to me) that I just didn't feel good and had low energy. 3 days later I had my positive test.
However, you are only 6 DPO and *usually* implantation is 8 DPO. BUT!!! I know some women do feel symptoms sooner and that may be the case with you. I sure hope it is that and *not* the flu!