Brooke, you're right. That's a pretty normal fluctuation that I wouldn't attribute to a BBT.
If it were me, I'd temp with your new one first thing in the morning... then temp again with the Walgreen's and see if there's any difference there.
I know there's something up with those Walgreen's BBTs though. There have just been too many girls having the same kind of issues and all using Walgreen's BBTs.
I hope this one works well for you.