Ready,
I agree with Emma... have a talk to your midwife/dr/ob at your next visit.
Basically, I think it would be a concern if the rapid weight gain continued... some people lose weight in their 1st trimester and regain it all in the 2nd and 3rd. In the end, most people catch up anyway.
I lost 8kgs in my 1st trimester, but I am slowly gaining it back now... but, I did have it there to lose in the first place, so as Emma said, because you were on the smaller side anyway, you most likely will gain more weight.