Linking from the PDF source posted by u/buzzzzzz (and linking from those links) is actually something of a rabbit hole. Here are a few. I haven't got time to copy and paste everything. Please note that there are other serious health complications also discussed:

Surveillance Updates of Myocarditis/Pericarditis and mRNA COVID-19 Vaccination in the FDA BEST System https://www.fda.gov/media/153090/download

Clinical Considerations: Myocarditis and Pericarditis after Receipt of mRNA COVID-19 Vaccines Among Adolescents and Young Adults https://www.cdc.gov/vaccines/covid-19/clinical-considerations/myocarditis.html

Coronavirus (COVID-19) Update: June 25, 2021 https://www.fda.gov/news-events/press-announcements/coronavirus-covid-19-update-june-25-2021