I'm a 26yr old female with a mirena (levonorgestrel). Had lower pelvic pain since aug. Had vag. Us today and it said a 2.7 x 3cm cyst. What needs to be done. It hurts?

Too long ! Ovarian cysts are normal occurrence . The ultrasound finding may not be the explanation to the pain. A follicular cyst (pre ovulation) will resolve and nsaids like Ibuprofen and Naproxen sodium will help. However, if you've had pain for almost one year you need a complete evaluation by your gynecologist including a laparoscopic examination.
Cysts will resolve. Small cysts on the ovary are very common even in young women who do not have an iud. The overwhelming majority will resolve on their own without surgery. Although they may hurt the best treatment is to take Ibuprofen for the pain and to wait to see if it goes away. Usually obgyns will repeat the sonogram 6-8 weeks later to confirm the cyst has resolved.
Pelvic pain not ok. 5-8% of women will have simple ovarian cysts related to the Mirena (levonorgestrel) iud. These are generally not worrisome. However, pelvic pain since august needs a more thorough evaluation. Ultrasound is a good start, but if your iud is positioned normally, it should not cause you chronic pain. You may want to try having it removed, but that may not solve the issue. You may need a diagnostic laparoscopy.