Having sex does not make you fat - in fact it can be quite good exercise! Whatever is causing her to put on weight, it's nothing to do with sex. Well, unless she's pregnant...
Are you making some connection between oral sex and her getting fat? Seriously, there is no connection there. There cannot be enough calories in a teaspoon of your fluid to make any difference to her weight.
Like Vance2335 says, the real solutions to putting on weight are diet and exercise. Maybe you could exercise together - go for a run together or go to a gym?
Has she changed her eating habits recently? Take-away food, restaurant food, and many supermarket instant meals can be more fattening that home-cooked food.
This is not a Dr's form, just regular people trying to help other people. So your girlfriend has started to get fat and you want her not to get fat?
My advice is if you love someone weight does not matter, you should be happy that you are with someone.
As for weight loss then she can go on a diet and exercise.
Is it possible that she is pregnant?