Cramping and Pain from IUD Insertion: This is the most common cause of pelvic and back pain after IUD insertion. The procedure can cause uterine cramping, which may radiate to the lower back. This pain is usually self-limiting and can be managed with over-the-counter pain medications.

Perforation of the Uterus: Although rare, perforation can occur during IUD insertion, leading to severe pain. The risk is higher in women with a retroverted uterus or those who are breastfeeding.
Infection: Bacterial infection can occur after IUD insertion, leading to pelvic inflammatory disease (PID), which causes pelvic and lower back pain.
Expulsion of the IUD: Partial or complete expulsion of the IUD can cause discomfort, pain, and bleeding, especially if the IUD is only partially inserted.

Ectopic Pregnancy: Although IUDs are effective in preventing pregnancy, if a pregnancy were to occur, it is more likely to be ectopic. Ectopic pregnancy can cause severe pelvic and back pain and is a medical emergency.
Ovarian Torsion: This is a rare but serious condition where the ovary twists around its ligaments, cutting off blood supply. It can cause severe pelvic pain and may be associated with IUD insertion, especially if the IUD has perforated the uterus and caused the ovary to twist.

Endometriosis: While not directly caused by IUD insertion, endometriosis can be exacerbated by the procedure, leading to increased pelvic and back pain in susceptible individuals.
Adhesions or Scar Tissue: Formation of adhesions or scar tissue following IUD insertion can cause chronic pelvic pain, although this is less common and typically develops over time rather than immediately after insertion.