What Causes Lower Abdomen Cramps And Dry Brown Blood Discharge?

i had my last mensturation on 25th july,had unprotected sex on 30july after which i bled,then again on 12th august i had unprotected sex,both the times we used pull out method,i have done 4 urine tests and 1 blood test,all negative, i had painful lower abdomen cramps for 4 5 days then they stopped, and now since 3 days i m experiencing dry brown blood when i wipe it,what should i do?plz help!can i be pregnant?