Experts are studying this now. No women in the clinical trials reported changes to their menstrual cycle after receiving the vaccine. Menstrual cycle changes are normal throughout the course of the year, and could be caused by stress, fatigue, or other lifestyle changes. If menstrual cycle changes persist, talk to your healthcare provider. You can also make a report using the V-safe After Vaccination Health Checker tool on your smartphone vsafe.cdc.gov. Harmful side effects can be reported through the Vaccine Adverse Event Reporting System (VAERS): https://vaers.hhs.gov/esub/index.jsp.