Hiya megan
when I was reading your post, I thought 'no, shes definately not pregnant, she can't be feeling that bad so early' etc, just because the probability of concieving during your period is very low, but like angel said its possible, and i've read quite a few times posts where people have said that they conceived during their af, so ok its possible you could be pregnant.
The nausea thing I can believe too, I too felt different and a little sick straightaway, I don't know how many days after I conceived that I started to feel different etc, because me and my hubby had been at it like rabbits for the week I was supposed to have ovulated, all I know is at some point during these days or a couple after, I felt a little nausea, and very strange, thought it just in my mind because it was our 1st month of trying (well 1st week hehe), didn't think I could get that lucky. Anyway as the days passed the nausea actually disappeared but that strange feeling inside stayed,and about 1 week before my af was due, I got veins around my boobs, etc, I tested the day before af was due and I got what I had deep down expected, and big !.
I think if your in tune with your body, u'll know if u are or not, maybe not now, as the nausea etc could well be due to other things.
Good luck !!! Xxxxx