If you are experiencing lower abdominal pain and the app indicates that you are three weeks pregnant, it is advisable to consult a doctor for further evaluation. While home pregnancy tests can be accurate, they are not infallible. There could be various reasons for your symptoms, such as an ectopic pregnancy or other medical conditions unrelated to pregnancy. A healthcare professional will be able to provide a more accurate diagnosis through physical examination and additional tests if necessary. It's important not to rely solely on the app's indication but seek medical advice for proper assessment and guidance.