Engagement talk

So my bf and I have been together for 4 years. Recently we've been really talking about getting married and having kids. Well for the whole month of july I'm watching his son because my bf works 5 hours away and the kids mom doesn't want him. His son let it slip that my bf wants to propose to me on my birthday which is at the end of the month. Now I'm not sure how I'm feeling. I love my bf and I want a future with him but why am I feeling so upset about it?