What Causes Delayed Periods Along With Abdominal Bloating And Cramps?

Hi I am 21 years old and was exactly 7 days late for my period with light cramps and a lot of bloating I do have IBS but my periods are always on time until now. I thought I was pregnant so I took two pregnancy tests and even a blood test from my doctors all tests came up negative. After the seventh day of my missed period I started my period and had the worst cramps so I took midol and about 2 hours later I got very nauseous I notice my bleeding is not very heavy like usual so I am very confused.