Contraception is never a sin in Islam. If you feel that you don’t want to have more children, it is your right to prevent pregnancy.
Please refer to the book From Marriage to Parenthood: The Heavenly Path especially this passage:
Consent between husband and wife
According to the legal aspect of Islamic law, the wife has full right to the use of contraceptives, even without the consent and approval of her husband. However, she should not use a method which may come in the way of her husband’s conjugal rights. For example, she cannot force him to use a condom or practice coitus interruptus. This rule is based upon the principle that the extent of the husband’s conjugal rights over his wife is just that she should be sexually available, responsive, and cooperative. This right does not extend to that of bearing children for him. Bearing children or not is a personal decision of the woman, and therefore, she may use contraceptives such as pills, injections or cleansing of the vagina after intercourse as they do not interfere with her husband’s conjugal rights.
Also the book Marriage and Morals in Islam:
According to the Shi 'ah fiqh, family planning as a private measure to space or regulate the family size for health or economic reasons is permissible. Neither is there any Qur'anic verse or hadith against birth control, nor is it wajib to have children in marriage. So basically, birth control would come under the category of ja'iz, lawful acts.
Here below you can read more about contraception in general on an Islamic point of view: