There are several healthcare options where you can get tested to find out why. Primary Care Provider (PCP) is a good first step. They can start with basic tests and refer you to a specialist if needed. A gynecologist (OB-GYN) specializes in reproductive health. They can do fertility assessments, hormone testing, and imaging (like ultrasounds).
Fertility Clinic / Reproductive Endocrinologist are specialists focused entirely on fertility and can provide a full range of testing and treatment options.