Changing nipple rings

Ok so I've had my nipples pierced since August 1st so it's been like 9 months. I've been pretty nervous to change them but I finally got new rings to change but I got hoops. I changed them last night and it was ok like it didn't hurt at all and it was pretty comfortable. The only thing that was difficult was getting the piece in to close the ring. But every once in a while they start to hurt pretty bad. It's not crazy bad but it's enough to make me uncomfortable. I was wondering if it's normal or if I may have tore something or if it's because of the type of ring. If anyone has any advice that would be awesome.