The doctor may have cut them short.. You need to feel your cervix opening.
The same thing happened to me. No strings! But I found out much earlier at my six week check up.
Here is what happened when they couldn't find the strings: They sent me for an ultrasound to check the location of the mirena. They said that it "appeared" to be in the uterus and told me that the strings had just gone up my cervix and that all was fine. So I left the darn thing in for a year. I occasionally had pain and other symptoms and requested to have it removed. The doctor tried to access the strings through my cervix but couldn't get them. I had another ultrasound and again was told that it was in my uterus. I then had to go for a hysteroscopy and D&C to get it out. It wasn't there! X-ray revealed it had perforated my uterus. I had to have it surgically removed through my abdomen.
I don't mean to scare you. Apparently perforation is very rare, but it did happen to me and it all started with missing strings! If I was you, I'd go to the doctor as soon as possible and ask to have it removed. In my opinion, it may be too risky to leave it in there, even if they tell you it is still in your uterus. Good luck!
In theory, they should be able to access the strings through your cervix. if the device is swimming around in there with the strings facing the wrong way, I presume they might have to dilate to get it situated correctly or replace it. I hope your doctor gets it back in place easily!