I found that getting an IUD has made PMS worse versus while on the pill, including bloating and overeating for that TOM. That alone may cause some weight gain. I've had mine for a while, I think it is fine, but I miss the days of minimal periods and minimal PMS that hormonal methods offered.
That said, pain outside of menstrual cramps and maybe some related to ovulation is something I have never experienced and could be a sign you have something amiss with your IUD. I'd get that looked at ASAP