Hi,
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The abdominal cramps and dark bleeding/discharge could indicate chances of infection. If the symptoms are getting worse, these could also suggest possibility of embedding of IUD or perforation. Do not ignore your symptoms. Visit your gynecologist at the earliest for thorough evaluation to look for the position of the IUD. Ultrasound scan done would help. If infection is suspected, vaginal swab/smear test done on the discharge would help to identify causative agent and get you started on appropriate treatment. Be in regular monitoring and follow-up of your treating doctor/gynecologist and report any new/abnormal symptoms immediately.
Take care.