It might be a good idea to wait a few more days... Do you know what day your period would be due on? Do you have a 28 day cycle? I do and my last period started on june 30th, I was due for my period on july 28th.. I missed it. If you have the same cycle as me, your period should of started on friday. Usually any time you test after a missed period, it will come up positive if your pregnant.
As for drinking alot of diet pepsi.. I dont think you have to stop drinking it, but you will definitely have to start drinking more water. I am the same way, I drink alot of regular coke lol, but hydration is really important while pregnant. I already had 2 cokes today... But here I am trying to chug down a 16.9 fl oz bottle of water haha.
Thank you for the congrats and yes, definitely keep us updated!
<3 jess