Welcome to the "oh jesus WTF am I doing?" club!
I felt exactly the same when I got pregnant - it happened very quickly for me too and I was fairly ambivalent about the whole thing anyway. I didn't feel ready at all, and I remember wishing it had taken months to get pregnant because I thought I might feel more excited then.
Do you have any specific fears, eg financial worries, or is it just general freaked-out-ness? If its the latter then you're perfectly normal I think!
My advice - first of all don't beat yourself up about feeling the way you do. Your fears are perfectly understandable. You are not alone in feeling this way. It'd be lovely for you if you felt all earth-mother about the whole thing but if you don't, it's not your fault. Don't compare yourself to other pregnant people / mothers who are more joyful about it, you will only make yourself feel inadequate. I remember wondering why everyone was excited about this pregnancy apart from me, and concluding that I was probably a horrible horrible person.
Don't underestimate the effect of pregnancy hormones and other pregnancy symptoms. I felt like shit for the first 4 months and that made it virtually impossible to be excited or happy. I HATED being pregnant at first! It does get better though. I'm quite delighted with myself nowadays (I'm 27 weeks), and excited about the pregnancy.
And don't look too far ahead. Don't worry about not being able to cope with a baby for example - this sounds trite but people do just cope.
Lastly, talk to your partner. It really does help. If you're freaked out about being pregnancy then it's a lonely place to be.
I think it's normal to question what on earth you're doing when you get pregnant for the first time. It's such a massive step and is totally scary! That said, there's no-one I know with kids who doesn't totally think it's worth it.. I have 3 step-kids and they can be an almighty pain, but I wouldn't be without them. My life has definitely changed for the better by having them in it - and hopefully having my own will be the icing on the cake.
Sorry for the long post - hope it's helpful. Good luck!