Mirena strings

I'm not able to feel my strings! I haven't been able to for quite a while now and I never really thought anything of it, but my period is late this month and I'm kinda worried. I had an ultrasound last summer and my doctor said it was still in place and everything is fine. Maybe I'm just paranoid because my period hasn't came on. Like I said, I haven't been able to feel them for a really long time. (A year or maybe even 2)