Almost 4 Years No Bleeding/now Spotting/cramping..

I'm a mother of three & have had Mirena for nearly 4 years. I stopped having my cycle almost immediately after it was implanted and never had any spotting or cramping after the first few days. Fast forward to now, almost 4 years later. I have been experiencing slight discomfort during intercourse (but it's intermittent), spotting a dark brownish discharge, cramping on and off that has doubled me over at times for the last month. I went to my GYN today and she couldn't see my string but claimed she could feel it. That seemed a bit suspect to me...BUT to her credit she put me on an antibiotic (although she said everything appeared healthy so that concerned me) and ordered a pelvic/transvaginal ultrasound. I had the ultrasound done today & now I have to wait to be notified of the results. I hadn't experienced any issues until this past month. I'm really concerned it has shifted and is not where it's intended to be...I just have this feeling that it's not though -__- Wish me luck!