I feel sick after they leave

I watch these three litter girls, age 6, 9 and 13 not often but a few times, like July 4th weekend I watched them from July 2nd to July 5th and they went home today and I miss them so much and I’m so bored and now I just feel sick, this happens everytime they leave, I don’t know why I always feel like this, it pretty much happens whenever I spend time with anyone for awhile and as soon as the leave I feel sick and wanna cry and want them back. I’ve known the 13 y/o since she was 3 and the other to before than were even born lol. I know many of you would not understand, I’m 21 years old and have no friends and that not a figure of speech actually have none, I’m always by myself and whenever I spend time with someone for awhile and than I get back to being alone I’m sad I’m very lonely I know in a few days I’ll get back to normal but right now ugh