What Causes Fluttering Sensation In Abdomen While On Depo Shot?

I gave birth in Oct. 2009 and in Nov. 2009 I received an IUD (Mirena). Around a month later I began to experience severe pain in my pelvic area and my lower stomach, followed by this fluttering sensation in my stomach, mostly in my left side and a few inches above my navel; in Dec. 2011 I had my IUD removed and received a Depo shot and the pain quickly subsided but the fluttering sensation still remains. I've taken numerous pregnancy test which all gave negative results. What does this mean?