I'm having pain into left lower side and is worse when I pee or sit down, could this be realted to my iud? Or gas? Or what could it be?

PID or UTI. I would be concerned about pelvic inflammatory disease (pid) or a urinary tract infection (uti). Both are infections caused by bacteria and need to be treated. You need to see a doctor now. They can determine the cause of your symptoms and start appropriate treatment. Pid can lead to infertility if untreated and untreated UTI can damage your kidneys.