I Usually Have A Regular Period Cycle Of 28 Days.

I usually have a regular period cycle of 28 days. On April 24 I began my period one day late but it was lighter and shorter than usual. Two weeks later on May 7th I spotted for 3 days and had some vomiting. On May 21st or 22nd I should have started my regular period but I have not started. I do not have any additional symptoms and the pregnancy test I took this morning was negative. What is the next step?