Pregnant possibility with IUD?

Patient: I’ve had my Mirena IUD for 2.5 years now and recently could not find my strings. They were intially cut short. I recently went in to my doctor and she was unable to find them as well. I have US set up in a few days to see if it can be located. I have not had my period since April of 2009 (Jan 2009 when got IUD).I have had sex with IUD in the past (few months back) and no worries of pregnancy, just recently with talking with physician thoughts prompted. When having sex though I do bleed, but do not get a period while being on Mirena. But recently had sex, and was wondering how long before finding out if pregnant and can I still get pregnant? I know that its still there, I occassionally have cramping and lower back pain, like I would if I was having my period. However it does not happen each month. I am not worried of being pregnant, but having a calm mind about it would be a relief all the same. Please respond with some sort of answer.Thank you.