Pain when riding

I’ve had sex 3 times now with my boyfriend and on the third time I tried getting on top (which I was excited to try). It was a little painful unlike the rest of the sex I’ve had with him and I’m not sure why. I feel comfortable and know the issue isn’t being with him. I had just gotten and IUD a two days before, so I’m hoping it was that and that’ll pass.

Was it painful for any of you when first trying to be on top? Are there certain angles that are better than others? Any advice welcome :)