What Causes Short And Light Periods?

i am 43 years old and for the past two years have had a period every 21-23 days and bled for up to seven days quite heavy flow. My last period was short only four days and light. i am now four days late, assume cant be pregnant as had my tubes tied in 2003. Had some cramp but nothing else.
i have had the odd night sweat and my moods are often up and down but no particular pattern. I have been on iron tablets for the last three weeks due to being anaemic and am currently undergoing some investigations for inflammatory bowel disease as yet not confirmed as it could be IBS
Can you offer any explanation please?