Question: My periods are getting shorter and I don't know why

My periods are getting shorter and I don't know why. They were always 5 days long and on time and now the last 2 months they were 4 days long and a day late and my most current one was 3 days long, 2 days late with severe cramps the first day. Has anyone else ever experienced this? Do you know what it's from? I've consulted my physician and she can't find anything wrong but this seems a bit odd to me.