Too many variables to say with any confidence.

I'd suggest OP weigh himself first thing in the morning, and then immediately when he gets back from the ride. Dehydration can make you feel dizzy, and this weighing can tell you if that's the case even before you pick up the phone to call the doc. Down a couple pounds? Make the call. Dropped 10 pounds? Have a (non-alcoholic) drink, perhaps with some salt (aka "electrolytes" if you want to throw some money towards an athletic store).

Note the normal advice is to do the pre-weighin right before starting the ride. I figure the 2 pounds he's drinking before starting should be counted as fluid intake associated with the ride.