If your dog is driving you crazy after being spayed here is the solution get a four inch ace bandage and a roll of four inch wide cotton rub the belly with neosporne place the cotton over the area and wrap the ace bandage around the dog but not tight when you get to the end of the wrap make it line up with the middle of the back and put on the clips it will be like night and day if you want any sleep.
Odd, I'm not sure why she would be itching like that either. There may be a slight chance she has an allergy to the material used for the stitches or something they used in that area on her skin, etc. I'd give the Vet a call and ask. Sounds like you're doing all you can do. I would ask about possibly trying the Benadryl. Just make sure the wound is clean and healing.
You can also talk to a professional breeder about it (like at http://www.cavachonsbydesign.com ). They know a lot of particulars about the specific breed.
I agree with ginger. I know that sometimes my wounds itch when they are healing, so it would make sense that the wounds on a cavachon would itch as well. If it gets too bad, I would take him back to the vet.
Wounds do often itch as they heal. Take a look at the wound. Does it look clean? (apart from fluff stuck as you said in the suture) Does it look at all reddened around the scar area, or inflamed? Is there any discharge from the wound, pinkish liquid, or straight blood? Is there any swelling or anything angry-looking around that area? (All of those things could indicate infection in the wound)
If not, presumably it is healing naturally. It sounds like you are doing your very best to keep her from scratching, but she's going to try and scratch anyway! You could ask the vet to be sure, maybe you could ring them and ask, but I would think that Benadryl would most likely help. However, I am not certain of the correct dose for her weight. I'm sure the vet can help you with this on the phone.
Maybe you could swap her regular dog-bed cover for a plain smooth cotton sheet, or something like that to prevent fluff getting stuck to the suture. My instinct would be to have two, and boil-wash each, every day, putting down a fresh one every day, to prevent the possibility of bacteria getting into the fresh wound?