A sudden decrease in menstruation does not necessarily mean that you are pregnant, but you need to have a test to confirm the diagnosis. If the amount is particularly low, it may also be pregnancy. You can test yourself with early pregnancy test paper or go to the hospital to check human chorionic gonadotropin to clearly diagnose whether you are pregnant or not. If the menstrual flow is lower than normal, it may be an endocrine disorder. Two tests can be done: 1) Gynecological ultrasound to see if there are abnormalities in the uterus, adnexa, endometrium, uterine fibroids or ovarian cysts; 2) Female hormone panel 6 to see if there are abnormalities in estrogen, progesterone, follicle stimulating hormone and luteinizing hormone. It may also be an endocrine abnormality caused by lack of rest, or stressful work or study, causing irregular periods.