I know exactly what you mean... recently picked up a bike and have been riding maybe 30 miles a week on the weekends and noticed this Sunday when I got on the bike for a bit and my tailbone was sore. Weather was too good to pass up so I rode anyway.
I've read this before where people have said that it's normal to have some aches in the beginning as you're body isn't used to it. Hopefully that's true and I need to put in some more miles!