Does Stress Stop Ovulation?

My daughter is getting married in 3 weeks and has been completely stressed out for sometime . july was her last period. august she was diagnosed with gastrointestional virus and was very sick for 2 1/2 weeks. she's had some minor cramping but hasn't had a period. in september she took a pregnancy test (over the counter product) 2x neg. i've read that stress can stop ovulation and usually after stress level back to normal she should get back to a regular period. any thoughts