My 2 year old cat (whos passion is food of all kinds) has suddenly lost her appetite and desire to play. She seems down and sleeps most of the time. :(
I gave her a spot on flea treatment (bob martins - first time using it) 2 weeks ago but she still has flea dirt in her fur and is scratching. Her sister has been treated with the same product at the same time and seems fine.

What could be causing this?
I have ordered some frontline flea treatment which has worked in the past - would this be safe to administer straight away?

I'm grateful for any help you can give me :)

Well same thing with my cat skittles and sasha both them used to play but they only eat sleep and -.- but then they got tooken away because they had a nervouse break down and started biting and scratching :( well stay strong:)

I would get her checked out by the vet first.

I can't see a plain old case of the fleas causing this kind of reaction. (I don't know what a bad allergy to them might do though.) So I think the vet is in order.

As for "ordering" Frontline -- I would not order it online. Get it from the vet only.