Yes they can leave bumps, caused by scratching.
This is very effective, and will kill fleas and their eggs in the house.
No it shouldn’t be… just let the vet know and let them know when and what you put on her. Maybe just show them the empty box it came in so they can see the ingredients and know the expiration date. You can buy frontline as well through Amazon FRONTLINE Plus for Cats and Kittens (1.5 pounds and over) Flea and Tick Treatment, 3 Doses https://www.amazon.com/dp/B0002J1F76/ref=cm_sw_r_cp_api_i_XPASZHJ3ZPF17XP6J6ZY?_encoding=UTF8&psc=1… The key to this stuff working is having the vet actually show you how to put it on. Our vet showed us because if not applied right most will ho onto the fur and not enough on the skin (it won’t work then) …their skin and natural oils help get that stuff working. Our vet used a comb to part the fur high up on the neck and then applied to the skin… https://youtu.be/r-LkYwU_I2k You certainly don’t have to use a comb to expose the skin but I find it helps expose the skin easier then fingers. Definitely see what your vet suggests as they know best for your kitty :)
You can get the same stuff they sell at the Vet on Amazon for much cheaper. I get this one: http://www.amazon.com/Merial-Frontline-Control-Kittens-Doses/dp/B0002J1F76/ref=pd_bia_nav_t_1?ie=UTF8&refRID=11X7XEGZ3RWJV7J343AS