Self check for strings

Ok I have my mirena put in at the end of November on December 8th I started my period and had a 20 day period my dr told me to do self checks for my strings the first few months and I have I've always been able to feel them and yesterday after I stopped my period I did a self check and couldn't feel my strings. Worry or is it normal for me not to be able to feel them every self check ?