Isa-
I can't imagine a 12 pound weight loss from a 139 frame isn't noticeable. I think that if you aren't that overweight to start with and you start losing weight, people are reluctant to say anything (unless you've already told them that you are dieting)- telling you that you look thinner now implies that they think you looked "fatter" before, and no one wants to tell someone that they need to lose weight (people are especially afraid that they might find out that they were wrong about you losing weight- they would think that you think they think that you are "fat"). Anyway, I don't think that came out clearly, but do you know what I mean?
Also, have you bought any new clothes that fit better (or broken out any "skinny" clothes that have been in storage). If you are still wearing your old baggier clothes, it might be hard for people to see the difference. If you haven't, wear something a bit more fitted- I bet someone will notice your loss, and if not you can just feel good about how great you look in it! It really is worth it to buy some smaller sizes (even if you still plan to lose more)- it feels wonderful
Jess