• Blocked or damaged fallopian tubes
• Ovulatory problems
• Hormonal disorders
• Polycystic Ovary Syndrome, PCOS
• Endometrioses
• Being over 38, from which age fertility declines rapidly
• Medical conditions such as diabetes, epilepsy, and thyroid and bowel diseases
• Lifestyle factors such as stress, being overweight or underweight, alcohol misuse and smoking
• Environmental toxins including exposure to pesticides and lead
• Excessive physical or emotional stress that results in amenorrhea (absent periods)

Well, it's not exactly your fault. Here's a look at the infertility rates : 

The best way to find out the cause is to get yourself checked by a fertility specialist or gynaecologist. Just call and make an appointment with them