Girlfriend Menstruation Problems, Taken a Pregnancy Test Negative

Hello everybody, I wanted to talk about my girlfriend, she is 18 years old, I was the first one who took her virginity in April, and she had regular period cycle on the 28th of every month, but last month in May it came May 5th, 2012, 7 days later, and I'm guessing it's probably the sexual intercourse that started in April.  Anyways, we had unprotected sex twice between May and June, one time in Mid-May, the last time May 26th 2012, I never ejaculated inside her, but I am aware of pre-cum.  We have taken a accurate pregnancy test on JUNE 6TH, 2012 and has shown NEGATIVE.  She hasn't had her period yet, HOWEVER, today is June 11, 2012 and she did experience this morning, heavy bleeding, but all of a sudden GONE.  She has extreme cramping/pain and that really bothers her and she requires lots of rest.  She has been stressing the whole month of May and just recently because we've been arguing alot and she's worried she's pregnant and about her father who is currently away on job and experiencing pain.  I've read that stress can impact ovulation process greatly even delaying for a month, so please I need to speak to women who have experienced these exact circumstances, please I hope she's not pregnant, but if she is I am willing to be the best father and be there for her... I just want to know what's going on, is this a sign of delayed period because of stress? pregnancy? miscarriage? We are set to take another Pregnancy test June 13th, PLEASE WOMEN WITH EXPERIENCES ONLY OR COUPLES THAT HAVE EXPERIENCED THIS OR ANY KNOWLEDGEABLE INFORMATION WILL HELP GREATLY. PLEASE RESPOND.