V.P.
I am very sensitive to changes in my body. I had tingling in my breasts within days of conception and that metallic taste. I also "just knew." Couldn't tell you why. I just did. I've only been pregnant twice, and both times I knew long before I could test.
By the way, I also "just knew" the gender of both babies. I was right both times. Not very scientific or statistically valid, but I "just knew."