Every STI test has a 'window period' - the gap between exposure and when the test can reliably detect the infection. Testing during this window can give a false-negative result even if you are infected.
As a rough guide: chlamydia and gonorrhoea are often detectable about 1-2 weeks after exposure; HIV using a modern fourth-generation (antigen/antibody) test is usually reliable by about 4-6 weeks, with 90 days giving a conclusive result; syphilis antibodies may take up to 12 weeks to show; hepatitis B and C also have window periods of weeks to months. Window periods vary by the specific test used.
If you have had a recent risk and a test comes back negative early, your clinician may recommend repeat testing after the window has passed. A licensed doctor can tell you exactly when to test and re-test based on your exposure date and the tests available in your region. If you have symptoms or a known exposure to HIV, seek care promptly - emergency HIV prevention (PEP) is time-critical and only effective if started within 72 hours.