Becoming a Father, with someone I didn't expect, Family is fine and hers doesn't know

From what I remember......

Looking after the baby whilst people are running around a house. I know the house belongs to someone in my family. My maternal Grandma specifically I know she's in the house with my mother in the kitchen or something. I'm holding my apparent son and playing with him making him laugh looking after him etc. I know the baby's mother she pops in and out occasionally picking up bags etc talking to my family (Father and Sister) who are with me. Her ex boyfriend is there, sulking or being quiet, I am of the opinion their back together but don't understand why.

Amiee (the baby's mother) has the same phone as me so I answer her phone thinking its mine. Her parents know she's with me but are a bit surprised when I answer. I go along with it after realising my mistake. They are sending over a car full of things that need unpacking then they want Aimee to clean it. I agree have a chat an hang up, whilst conscious of the fact I'm holding a baby and they don't know Aimee has had it. I walk upstairs (still holding the baby, making him laugh etc) to hear Aimee crying from the bathroom. Not know what to do I walk down, make a remark to my Mum who looks concerned then go stand at the doorway. Aimee appears and I relay the conversation. I then continue playing with the baby whilst Sep (the ex boyfriend) my Dad and Sister unload the Car.
I walk out and Amiee asks me to help her clean it. I hand over the baby to my reluctant sister and know that my Dad is looking at me the entire time, I avoid looking at him. I walk over to Amiee and ask her if she's sure the child is mine. She says it was either mine or Sep's and that Sep was certain it wasn't his. So she thinks it is mine because we had drunken sex after her and Sep had broken up, I was consoling her; one thing led to another. I ask about her family at which she says they don't know and she doesn't know what to do. She says her Dad is going kill her and give her loads of hassle about it, I reply with 'my Dad is already hassling me'.
We start cleaning Car and I remember Sep, who I now know is obviously very angry at me. I feel bad and want to ask him about his certainty but feel I already know the answer.

Long winded I know, but this dream really shook me. I loved it, especially when playing with the baby, and at the same time woke up feeling quite shaken and apprehensive. I did have to reassure myself it was a dream for a second.
I'm really not sure what it means. I know the girl I had the baby with, I worked with her and knew a bit about her life, hence the ex. But I don't understand why it was her I had a baby with. I also get that this has to do with my relationship with my parents. But can't understand the rest of it, like why my family is fine with this or why I don't appear to have an argument with Aimee for not knowing about the baby.
Any replies are most welcome.