Anyone else with no period for months & not pregnant?

So I haven't had my period for 4 months. I took a pregnancy test multiple times & all came out negative. I wasn't on birth control at the time and I wasn't normally irregular. I just got the Skyla in and a few weeks later finally started. But I was told if I didn't have a period for another month to go back to my doctor for X-rays ect. It didn't come to that thank god. But still wondering why I didn't have them all those months? Has this happened to anyone before?