Some women do find that it may take a few tries before getting used to wearing Uresta, and may experience mild pelvic discomfort. Often this is due to tension in the pelvic floor muscles, which generally relaxes after a few uses.
Have you tried using a slightly smaller size? You could always try a smaller size for a few days or week to allow your body to adjust, before trying the next size up.
If you have experienced pelvic floor tension in the past, it could be worth consulting a pelvic floor physiotherapist to help relieve the hypertension.