Congratulations, I didn't realise you are soon to be a mother.
So sorry you had this attitude come at you, that is very rude of the woman.
When I was in my early 20s I used to take my baby niece out and people would think she was mine, I also looked younger so some people thought I was a teen mother - I had a few unkind comments. One guy said to his friend "What a waste these young girls having babies", I just thought it was hilarious.
Parenthood has it's challenges whatever our ages and circumstances, I don't think any of us can say we are truly prepared for it. As long as you put your daughter first and seek help if and when you need it you will be okay.