Some dogs, as Dixieland Dancer says, do have a "heavy shed" in both the spring (to get rid of the winter coat) and fall - to prepare to grow the heavier winetr coat. I'd just brush and brush, and see what happens in a week or so. Border Collies havea pretty dense coat in the winetr, so I wouldn't be surprised if this is what's happening. Does the skin (if you can find it ) look nice and healthy?