Hello
Femilon is an oral 
contraceptive pill , it contains a combination ( desogestrel+
ethinylestradiol ) of hormones that help prevent 
pregnancy . 
After delivery due to dietary habit , usually most of female gain weight , so same is in your case . If you want decrease weight avoid spicy , fried, oily foods, also avoid pizza, burger sweets , do regular exercise . 
As far as concerned about the side effects of this drug , these are :
Intermenstrual bleeding, post-medication amenorrhoea, changes in cervical secretion, aggravation of endometrium secretions and vaginal infections. Nausea, vomiting, mood changes, 
depression , CHANGES IN WEIGHT , fluid retention. 
These are rare side effects and usually  females can tolerate easily and the drug adjust after some cycles . 
But for weight gain , you should do as mentioned above.
Otherwise drug is safe.