FLEAS !!!!!!!

Hi All, Hilary & Socks here again....looking for advice......Socks is scratching,fleas suspected!!!ONLY problem is.....the 6 months of frontline I bought from vet (very expensive) states that if cat is not well then perhaps do not treat......Socks has,what the vet calls recurring ´cat flu´,possible stress related and probably always will. Have bought a cat flea collar from Pets at Home yesterday and put it on her BUT within the hour it seemed to be affective her eyes and she started sneezing more,yawning and ´down´ not wanting to purr or play.!!! So took it off.
Don´t know what to try now.....would herbal drops/collars work effectively?.I really don´t want to risk anything that will upset her again (was poorly for 2 days after her 2nd vac).
Perhaps the effect on her is worse because she is small ?
Appreciate any advice from the wonderful expert animal lovers out there on Petstreet.
Apart from todays drama......she is enjoying her life here with us and we are so enjoying watching her.The SUN is also out again.......enjoy !!!!!!!