I too was stung by a bee(?) 5 miles into a 66 mile ride. As I was riding down a fairly steep hill, a bee stung me on my abdomen through my jersey. The thought of and pain of the sting caused me to react by braking, I lost control of the bike and flew off onto the side of the road. After laying still for a minute or two making sure nothing was broken (me or the bike), I continued the ride.

The jersey rubbed against the sting which was very aggravating and the itchiness came later and lasted for at least a week if not more. I put hydrocortisone cream on the sting but it didn't really do a very good job of stopping the itch. I totally relate to wanting to scratch it for some temporary relief. I hope you're able to get beyond the itchiness sooner than I was.