C.G.
It is possible to get pregnant with an IUD but VERY rare. Antibiotics do not affect the IUD's effectiveness (the only reason they affect birth control pills in some cases is that they interfere with your body's metabolism of the drugs). Mirena has progesterone (levonorgestol I think) which is also a hormone of pregnancy so it may be that your symptoms are coming from the hormones in the IUD. Good luck.