I use K9 advantix II- 10 lbs and under ( and for pups 7 weeks and older). It's a topical, but your vet should have it. It works pretty well for mine so far. I've also used zodiac spray when needed in the past- puppy formula since most of my dogs have been small- 14 lbs and under. That kills eggs for up to 30 days.
If you've already used something for fleas, I'd ask the vet before using a different topical, tbh. Call them and ask.
Otherwise, hunt down that flea and kill it!