I stick to pet shampoos made specifically for small animals. Human shampoos (even baby shampoos) contain all sorts of chemicals that make it smell nice and lather well. These will leave a residue, even if you rinse really well, and this isn't designed to be ingested. That's fine for people, but guinea pigs lick their fur and will just end up eating whatever residue is left. Stick to shampoos that are made for them, and with their behaviours in mind.
When the seasons are changing, their rate of shedding usually increases as their coat adapts to the new weather. So that could explain the extra hair coming out.
Remember a couple of baths a year is plenty for a guinea pig. If their cage is kept nice and clean, they shouldn't need it any more than that. Most don't enjoy being bathed, and it can dry out their skin, so it's best to only do it when needed.