Are you testing/charting to know when ovulation happened?
I had symptoms VERY early, 16 days after the first day of my last period, which might be similar timing to you, but I wasn't testing and I can only think that I must have ovulated a bit early. Both pregnancies I had symptoms before I got a BFP (including nausea and a rash on my hands) so I guess I'm really sensitive to the hormones but I don't think that is the norm. I ended up with REALLY awful morning sickness so I wouldn't wish it on anyone else! I think it is much more likely your tummy was just a bit upset because of a bug or something.