What Causes A Delayed Menstrual Period With Negative Pregnancy Test?

Si, I am very worried about my monthly period. my last period was on 28th July. its 14th Aug now. i am not yet have the menses now. I have a 2 years old child now. So i don t want to get pregnant now. I tested it with a pregnancy card but it shows negative but still am very disturbed. Please advise me to avoid this if it is there. can you please suggest me some medicine to avoid my fear. I am not in a situation to have a baby now. Please help me to avoid this situation.