Chill feeling on my forearms while riding in 90s degree heat?

White

Probable cause is dehydration. Chills are a symptom of dehydration. On a 100K if not drinking about one bottle per hour, on a 90 degree day, could easily be the cause. But, as you identified, clearly bonking goes hand-in-hand with your ride. If you have ever bonked, very strange things happen!

In addition to your normal nutrients be sure to take GU energy gels with you. They are light, and a lot of them fit in a jersey pocket. They provide that instant hit of energy that you may need. One an hour will stave off the demons. On rides of 50 miles you need to take some form of energy drink... and drink about one bottle of liquid per hour on such a hot day.

Soccerref

Was that the only effect you noticed? Was there also any numbness or cramping? Often when you ride longer distances and you are tired you forget to move your hands around on the bars, which can cause numbness. How long did it take for the chilliness to go away after you finished. If it went away quickly, this would be my guess.

The other obvious possibilities are dehydration and bonking. Dehydration is an obvious answer but it is usually accompanied with a lot of other miseries besides cold arms. Bonking is simply running out of gas (consumed nutrition) and often happens at about 60 miles (3-4 hours) and also has a lot of general misery with it, usually lightheadedness and dizziness.

While I've never heard of this very specific symptom it falls in line with the general results from these problems.

If there were absolutely no other symptoms you should see a neurologist.

2 reasons:

1. Sweat evaporates quickly while you're riding, chilling your skin

2. After a a few hours of hard effort, you're probably dehydrated. That can mess up your internal temperature regulation. Its one of the beginning symptoms of heat exhaustion. Keep up with your fluid intake. At those temperatures, you may need to drink as much as 2 liters per hour to keep up.