What Causes Delay In Periods ?

Hi, I am prerna. My age is 21. I used to have my periods during the 1st week of a month. I had sex wid my partner on 21st december. But till today i dint have my periods. I had some spottings during 14th january but dat was not period. Am i pregnant? If yes, how can i abort it without going to a doctor? Plz tel me d name of a medicine which i can get from the medical store widout prescription