Can't feel IUD strings or cervix?

I got my Mirena IUD put in on December 21st and I've been checking my strings how my doctor told me to. I went to check it last night and couldn't feel my strings. I felt something smooth and spongy that could've been my cervix but I'm not sure because it was only a couple of inches away from my vaginal opening. I've been feeling light cramping on the right side of my pubic area. Should I call my doctor? I'm unsure of what I should do. Help!