Could you be pregnant if you had a delay of period by 18 days and then had a normal period felt like was going to get the period at proper time but it did not come and 7 urine tests were negative?

Also had vomiting and indigestion and dirreoha last week that is the week after the normal period. Even my exam is going on so slept just for 4 hours each day so i reckoned its because of that but want to confirm. Also did not hav sex but can precum on fingers hav caused this? I am really tensed so people please help me.