Why is my cat constantly licking and scratching herself during her pregnancy?

hi i have a very heavily pregnant cat. and she constantly licking and scratching herself. i was told before she was pregnant she had an allergy to flea bites and she has to be flea-ed once a month so they gave me some flea treatment. i have checked her and there is no sign of flea's any where on her. Why is she doing this if there are no flea's on her body? Please help! Thanks..