What Causes Missed Period While UPT Is Negative?

i am 20 years old married. last month we had unprotected sex and i didnt get my periods. i did urine pregency test which was negetive. still my husband bought me some medicine by which i got my periods. i got my periods on 30th of june. now i am not getting my periods. i did urine pregency test today and still it was negative. what should i do? we dont want a baby now.