What Causes Abdominal Pain,dizziness And Nausea After Periods?

i had unprotected sex the 2nd and 3rd of january using the with drawl method i had my cylce 4 days later and stayed on more than 7 days then this month feburary my cycle came late and im havin lower abdominal pains, lower back pains, feeling nausous and for the first time felt dizzy could i be pregnant i took a pregnancy test it was negative.