this is a really tough situation. i have had a few experiences like this in my life and from my observation, sometimes it can be difficult to be a group of three. if two people just want to hang out together, invariably the third one feels left out and rejected. not fun.
it seems to me that julia and kristen have developed a close friendship. since you were initially very close to both of them, it is natural for you to feel left out when they want to hang out together sans you. i can't say if they are purposely excluding you to be mean or if they just want to hang out and get to know each other better alone.
there are a few things you can do.
1) you can tell julia and kristen that you feel left out when they exclude you from hanging out. you can also tell them that it really hurts you to find out that they are doing this behind your back and being dishonest about their whereabouts (i.e. the excuses about not being being able to get together with you and then you finding out they are with each other). they may honestly not think this bothers you but if they are trying to hide it from you then chances are they know that they are being unfair to you. BUT to be sure, you need to tell them how you feel. It doesn't have to be accusatory or confrontational. You could speak to each of them separately. But get your feelings out in the open. Bottling them up inside won't make you feel better.
2)On occasion that you find out through Julia's mom (or whoever) that Kristen really wasn't delivering food to the elderly (and thus could not go to the mall with you, for example), you could find the two of them, catch them hanging out together sans you red-handed, and confront them. This option probably will not produce favorable results however, so I advise against it if you are truly interested in preserving these relationships.
3) You could take this as a sign that the two of them have become very close (maybe best friends) and are subtly trying to phase you out of the circle of friends. I agree that this is crappy and certainly hurtful and unfair, but you can't control other people's actions. At this point, I would concentrate on making some new friends or developing other relationships with other people. Keep yourself busy and surround yourself with people who want to hang out with you and share your interests.
You didn't say how old you are but as people grow into their teen and young adult years, they change their interests and oftentimes, this accompanies a change in the people we surround ourselves with. Perhaps Julia and Kristen don't feel as close to you anymore but have developed a closer friendship with each other. It hurts but people come into your life and also move out of it for various reasons. That is just how it goes.
Sitting in your room being upset with these two and feeling like you have a sad, pathetic life is not going to make the situation better. Plus, if Julia and Kristen think you are sulking in your room over them excluding you, then do you think that will make them want to hang out with you again? Perhaps you are making yourself too available to them. Get out and have fun with other people. My best suggestion would be to get out and meet some new people or hang out with some other friends you may not be as close with and concentrate on developing those friendships. Other people do not have the power to truly make us happy or sad unless we let them. It is up to us, and us alone, to choose to be happy or depressed with our current situation. Therefore, it is also up to us to make the necessary changes to get ourselves to the places we want to be in this life.
Good luck sweetie.