Hello, If you are having regular menstrual cycles, then as per your dates gestational sac should be visible when you had the ultrasound. Yet, sometimes due to low visualization, errors may be made, so it is always better to repeat the ultrasound when in doubt. Also, hCG levels do not fall so drastically after a miscarriage. I suggest you to take the test again. Adenomyosis may lead to infertility but the role it plays in pregnancy is not quite clear. I advice you to consult your doctor for a clinical correlation and further management. Good luck.