If i'm having pregnancy symptoms like lower back pain, constipation , and nausea and on mirena (levonorgestrel) bc , could i still be pregnant ?

Not likely: Getting pregnant on Mirena (levonorgestrel) doesn't happen very often. Constipation and low back pain aren't common early pregnancy symptoms. There are many causes for nausea. If you're concerned check a pregnancy test.

Yes: Very rare, but possible. Check a pregnancy test and call your doctor right away if it is positive. If negative, these could be symptoms related to the Mirena (levonorgestrel) or hormone changes during your cycle, so still a good idea to check with your gyn.