How does stress affect your period?

Last month was my first full month off of birth control. This past month has been amazingly stressful; so stressful it may in fact have caused an inflammation in my small intestine. I went for a colonoscopy this past week, and im only 20! My period is about 4 days late now. I took 2 home pregnancy tests and they were negative, but sometimes I get a pressure on my lower left hipbone (i heard this was a sign of pregnancy) and am having irregular stool....I have never skipped a period before, and am usually on time. Are the take home tests wrong, or is my stress altering my period as well as my stomach now?