Think I may have a poorly cat, or maybe just frightened, not sure, help please!

I have two female cats, sisters, around 1.5years old.  In the last couple of weeks, one has started isolating herself, staying downstairs all the time. 

I thought she may have worms so gave her worm tablets in her food and have treated her with flea treatment as she has scabs on her from where she is scratching.  I have treated the house with flea spray and vacuum regularly.  I have combed her for fleas and only get the odd one off her but she scratches a lot - much more than her sister - and constantly has lots of scabs on her which is distressing for me and no doubt for her too. 

She is still staying isolated and when I try to pick her up off the settee she's on, she puts all her claws in and gets quite scared.  When I manage to pick her up, she wraps her front paws around my hands and clings on, but not with her claws.  Once I have brought her upstairs she becomes more friendly again but she mainly stays downstairs on the same spot on the chair.  When I stroke her she purrs, and she responds to my voice. 

She is eating, but I don't know if she is using the litter tray as I have two cats and they both use the same tray. 

Any advice?  Becoming increasingly concerned and don't know if I should take her to the vets.  I don't want to waste their time and distress her for no reason, but if you think it's something that she needs medical attention for then I will take her immediately.

Many thanks.

EDIT: She originally had a little bit of bright red blood and mucus in her poo (I assume it was her and not her sister who has been behaving normally) which was a mixture of soft/normal. As soon as I noticed this I treated them both for worms and have not noticed any more blood.

Update:

Took her to the vet yesterday afternoon and they confirmed that she is having an allergic reaction to flea saliva.  They put more flea stuff on her (Advantage which he said was stronger and better than what I had been using - Bob Martin), and some steroid tablets to stop the itching.  She's already perkier and happier so I am really pleased

I have a cat with flea allergies and usually after a steroid shot and antibiotics she's good. It took me sometime to get ahead of the fleas. At some point you may find the advantage may stop working due to flea resistance this is normal just switch to another topical like frontline or revolution. Mine is on revolution. Once you get ahead of the fleas your kitty will be fine.