I had a similar experience on a hot and hilly 70-miler. I drank about 3.5 litres but came back weighing 3 kgs less, which implies a net fluid loss of about 3 litres. I.e. I must have sweated out about 6.5 litres on the one ride, having not peed the whole time!
I drank 2 or 3 litres when I got home but didn't pee until the next day. Oh, and I was salt-encrusted too!