I did the Clifton loop on Saturday and rode with some friends on Sunday on a Bristow loop (~50 miles). I'm getting more confident on the bike and with various types of roads. Now if I could just find the most comfortable grip position for my right hand...
Most likely it's just a muscle buildup thing that'll go away quickly. Also, don't overgrip the bar, which is a common thing. It'll all come to you with a little more experience. Good thing is, practice is fun with these things.