Agreed. I think its probably BOTH muscles swelling and water retention. I went on a long hike last weekend, and drank my whole 100oz camelbak and ate sugar/salt foods to keep my electrolytes up, and after showering upon return, could barely button my jeans. Next day - totally fine.
2011 Specialized Dolce Elite/stock Riva
2010 Jamis Durango 1 Femme
2006 Diamondback Wildwood (gift from my mother-in-law, so I must keep it! It is really comfy to ride)
1996 Raleigh M-20 (got my love for cycling going!)