Issue with iMessages

so here is the deal, my boyfriend and his mom share an Apple ID they have shared it as long as they can remember and for six months I haven’t had this issue. the other day I updated my phone and that is when I started having this issue any time I try to send his mom a message I go to her contact and select send message every time it brings me to my boyfriends messages. If she sends me a message it changes the title of the messages to her name but his messages are still in the box. It is really annoying someone please help me quick as you can!! And if it matters I have an iPhone 6