18 days ago i had sex .Ive been sick with naseau, very sore breasts, fatigue, backaches, acne, starting about a little under a week after it happened. ive taken tests starting at around 8 days after on and off until yesterday and all are negative. i havent taken a first response or really good test in like 10 days.. i took a dollar one yesterday with second urine and i had not peed in a few hours or drank anything and it was very concentrated and the test was negative. could i still be pregnant? i should have got my period a few days ago and haven't.. i get it every three weeks since i first started getting my period around age 12