Wow! Congratulations on the proposal first off! I'm incredibly happy for you to hear that
In regards to your question though I unfortunately can't speak from experience myself but I can speak logically (from my perspective). It's like this.. take a minute and actually realize what happened, you got engaged! From my end, I'm just in a normal relationship but you took it to the next level. You decided that she was the one and you both have taken a serious step forward in life. One that, quite honestly, will change your lives forever. So is it normal to feel nervous after something of this magnitude? Absolutely.
Quite honestly, just imagining it myself I'd be so overwhelmed with emotion I'd throw up lol.
I wouldn't go off the deep end and start thinking that your nervousness might mean you "secretly" didn't want the proposal to happen. Like I said above it is most likely because of the huge life-changing event that has happened and reality is settling in
Again though, congratulations on the proposal! Bless you two