Period?

So I got the nexplanon implant on 3/20/15 and I finished that period on the 3/21/15 but since then I haven't had a period since. For 6months I had no blood or cramps or nothing and now I get some spotting here and there but nothing really massive. I am use to my periods being very irregular but I am concerned. I went to my doctor and they didn't know what to think and I am just curious if anyone else has had this same thing or anything different. Thanks in advance