Need advice on baby jealousy!?!?

So we have one child. She’s almost two. We have noticed she’s not really a people person when it comes to children her age..... that’s okay I wasn’t either but the children love her so I’m not too worried. However, dropping her off this morning at daycare they had merged the T1 and infant class (ones crawlers other are walkers) so then a newborn/infant came in and their teacher wasn’t there yet. My child went in walking perfectly fine walked in her class like a boss until her teacher got the newborn since the mother was in a hurry and we couldn’t find her teacher. Well I of course (having baby fever and all and talking about another one with my fiancé) go awh she’s precious and (staying my distance out of respect of course) ask how old she was. The mother said 7 weeks and my baby’s teacher and I both coo and are gushing over the baby’s size she’s so tiny! Also making the mom feel good and important.... well my child immediately goes crazy crying wanting me I of course pick her up and the newborn is getting upset so I shsss mine like “baby it’s okay we have to use our inside voice look she’s tiny and getting upset we need to be a big girl and show her everything’s okay!” And I move and her teacher moves to show the baby’s now red face. Well of course my child automatically curls up and grunts (displeased) with a look of disgust on her face..... I of course apologize to the mom and the teacher and weakly say “well I guess that answers our question of another baby” and they chuckle (this all happened within minutes of arriving don’t think we were standing there for 5 minutes this happened within 2 minutes) I’m just a little worried because she has only child syndrome real bad!!!! Does it go away? She’s real territorial of any family when they are playing with a dog or child and I know it’s all of our faults because she’s been the only child on both sides for two years. Now she has a cousin and she wasn’t interested in her in the slightest. I’m just worried it’ll be a mistake to have another one without traumatizing her. Maybe we need to wait a year or two after the wedding.... will it go away or get worse? I know a child that got really worse and they were having a lot of issues with and had to re train her using the potty . Anybody have advice?