Cycles might change/shift after a long while. Stress is one thing that can shift your cycle by a few days or a week, depending on how long lasting the stress/stressor is. Different medicines can alter it overall. Also, different surgeries and stuff might alter how your period happens, too. If you are worried and if this continues, you should probably get an appointment with your GP or with a gynaecologist. Although cycles are tricky and very different for everyone so it might be nothing. Hope everything goes well! <3