Thanks for the query.
Some amount of menstrual irregularity is common while being on intrauterine devices.
Brown color of the blood indicates collected blood from the endometrium.
Abdominal cramps, bloating
sensation etc can also be seen with intrauterine device.
Possibly these will subside soon.
If you are near your expected date of periods, this can be considered as withdrawal bleeding, if not this could be intermenstrual spotting.
If you are having any associated symptoms like white discharge etc, better to get examined once to rule out the possibility of pelvic infection
If the spotting persists, you can take additional dose of hormones to control it with your doctor's advice.