I wouldn't be freaked out at all, some of the girls in my PAML thread are feeling movement and are able to see it from the outside and they are at about the 18 week mark too. With my DD I was getting quite strong movement by this stage and she was my first baby. This time I have an anterior placenta so movements are much softer and few and far between.

Because you're a smaller person your baby doesn't have as much room to float around in there and you will probably 'feel' your prg earlier in general - if that makes sense??