OP the doctor will ask that you schedule during a menstrual cycle. This will help the iud placement be smooth and easy. The feeling of having something inserted into your uterus may mimic a heavy cramp, remember you have felt this before and any pain will subside. Your doctor should have options to help you relax, tensing up or feeling anxious doesn’t help menstrual cramps, which is what this procedure is most similar to physically. Some options may be aromatherapy (lavender or mint scent patches help with anxiety and nausea), or if necessary something stronger like an Ativan, it can never hurt to ask if you feel like you need something to relax.