A question for Orchestra people

[NatureForever]

I'm playing Vivaldi's Four Seasons Suite in a Quartet with a few buddies just for the fun of it, and I've noticed that my A-string gets really sensitive in 5th position or higher, which is weird because my Viola has a clean set of everything. However, it's only when I take extremely short, repetitive bowstrokes (Thank you, Vivaldi) that it gets that way. The people I play with agree that my instrument is fine, and I'm pretty sure I'm doing everything right. Any ideas what the problem might be?