Pregnant cat wont stop over grooming.?

I found a stray cat about 12 weeks ago, which we thought was pregnant, she hasnt had them yet as the vet said she had 3 weeks left of her pregnancy, 4 weeks have since passed, and no kittens! The problem is she is over grooming herself constantly! which is making her skin sore, and scabby. Are there any treatments i can buy to help this? She going back to the vet next week but i would like to try help stop the over grooming before this as her skin looks awful, she also pulling her hair out which i no is normal for a pregnant cat?