Bride-to-be jitters - normal?

my fiance and i have been together for almost 3 years, we are both out of college and recently bought a house together (4 months ago actually). and he just recently proposed to which i said yes. i love him and we planned to get married and have kids so thats not the issue but ever since i have been having panic attacks and feel like everything is moving too quickly. i am 23 and usually very laid back so this concerns me. are these feelings normal? for a very long time i told him i was not ready to get engaged then when i finally said i was and he proposed its like i actually wasnt ready (or so i feel). we have only been engaged 3 weeks. whats wrong with me he is a great guy. please someone give me some advice i feel like a terrible person =(