I wonder why the pain just started now.
Is it muscle pain, joint pain or something else?
When you say you "try" not to lock your elbows, are you catching yourself doing that anyhow? Is that happening more now than it used to?
What makes you sure it's not the cold weather? Do you live someplace where it's colder this time of year than earlier in the year (even if it isn't "cold" by other people's standards?) Are you being diligent about keeping your neck warm? Lots of people hunch their shoulders when their necks are cold, which causes all kinds of problems with the upper body muscles and joints. Or if your chest is cold, you could be drawing your shoulders inward.
Did the pain start when you increased your mileage? Are you riding in different places, or on a trainer? Did you slack off on your core work over the summer?
Speed comes from what you put behind you. - Judi Ketteler