Very probably to lavender oil.
Thank you so much for your help, although to be honest, I didn't quite understand some of it. My guineas are swelling from Lavender?
A contact reaction to the new bedding is certainly possible; the lavender or other additives may be the cause. I recommend immediate removal of the bedding.
I found this info from http://www.dermatitisfacts.com/allergic-contact-dermatitis/allergic-contact-dermatitis-herbal-extracts.php
Lavender oil is extracted, for its fragrance, from the purplish flowers of lavender, which belongs to a family of aromatic Old World plants of the genus Lavandul. Two contact dermatitis cases have been reported from use of lavender oil. One was due to sleeping on lavender-scented pillows. The other was a case of acute facial dermatitis in a physiotherapist, due to application of lavender scented gel. Patch testing showed positive reaction to lavender. Fragrances and therapy oils containing lavender have been reported to cause contact dermatitis.