Bailey, Emily had a small umbilical hernia as well. Her's stayed out pretty much all the time but went out further when she was upset/crying. We were told the same, that its not an issue and they often take care of themselves.... Just to watch for signs of it being stuck out. Her's has actually already started to go in on its own... It is barely there now and mostly looks "normal!" We didn't expect it to repair itself this early, but it is certainly doing so! It got worse a bit after her cord fell off and then started to get better a few weeks ago when she started to be sat/stood up (she, obviously, isn't sitting or standing on her own yet) more and wiggling around more.
Hopefully Millie's will do,the same!