This not much of a surprise here, but sickness is associated with the use of commercial flea and tick products in dogs and cats. The chemical used was 9.0% fenvalerate, a synthetic pyrethroid insecticide which was suspected to cause illness. You can read the article from PubMed here.
"Oral dosing of dogs and cats produced severe clinical illness at doses as low as 0.66% of a can (7 ounce spray can)/kg body weight. Dermal application of the product resulted in minor clinical abnormalities in dogs. Oral exposure at 0.5% can/kg body weight resulted in severe illness, and dermal application caused severe illness or death in cats at 20% and 40% of a can/kg body weight. The cats receiving 10% of a can/kg body weight dermally became depressed for several hours but recovered uneventfully."
Essentially, exposure caused sickness. This article was written in 1991. Just because something is legal or deemed safe to use, doesn't mean it actually is.
More recently, active ingredients including fipronil and methoprene, used in many common flea and tick medications such as PetArmor Plus, Sentry, Frontline, Advantage II and K9 Advantix have also been linked to sickness. The use of fipronil on dogs resulted "in skin irritation, lethargy, incoordination, dilated pupils, facial swelling and convulsions. In laboratory animals, the chemical causes aggressive behavior, kidney damage, and changes in thyroid functioning. Fipronil has a long residual time, and can remain on pets nearly two months after an application. Human exposure is associated with neurotoxicity, kidney and liver damage, skin irritation, eye damage, reproductive impacts and cancer" (Read more here).
Instead of using flea and tick medication that is linked to sickness, you can use a product called wondercide (linked here) that is completely safe, natural and without potential harmful side effects. If your dog is going into a heavily wooded area, put a t-shirt on him or her so exposure to fleas/ticks are lessened.