I live in south Louisiana, fleas are a big problem here, especially since I live in an apartment complex with almost all dog owners. I originally started Toki on sentinel, she was on it for 2 months before I found a flea on her, I found out from the vet that it doesn't kill them so I put her comfortis about 2 weeks ago. I recently found a flea when brushing her. Can you get a random flea problem even though the pet is medicated?