The joint on my baby finger swelled up and has fluid inside (i had an ultrasound). it's been painful, red (but not warm) and swollen for about 3 months. it seems to hurt most in the morning. none of my other joints hurt?

This is just a single joint and from what I can gather, there seems to be no other complaints to suggest a systemic process such as RA given that this has been present for 3 months. If there was preceding trauma, one might consider a traumatic synovitis. If there has been or was a rash or fever, one might consider juvenile idiopathic arthritis or adult Still's disease, but this is unlikely.