Try telling your friends how you're feeling. They're not mind-readers; they might have noticed your problems but thought if you never bring it up, it's because you don't want to talk about it. Different people handle different problems in different ways - some of my friends like to tell people how they're feeling, others prefer just to talk about other things to forget about their problems.
Your problems are obviously very important to you, and although you may think it's obvious that you're going through a hard time, it might be that you're really good at carrying on as normal and you don't make it obvious to your friends. It's also possible that if they see you every day, they won't have noticed any minor changes in your appearance so this might be why they won't have noticed you've been losing weight.