K.,
You are not alone in feeling this way. I have been a single mom for a little over 2 years now and I sometimes feel the same way when I am alone with my son. One day I finally got sick of feeling that way and I decided to do something about it. The first thing that I did was find some other single moms that I could hang out with. I posted on some parenting forums that I was thinking of starting a playgroup for single moms and their kids. I also started talking to some of the moms at my son's daycare. I now have 2 other moms that I spend time with. It helps to have someone to talk to that knows exactly what you are going through. Just because you have a kid doesn't mean that your social life has to end. You just have to be willing to adjust it a little.
The second thing that helps me is keeping busy. When my son and I are alone, I make sure that I plan something (even just going to the park works) that we can do together. It's alot harder to be lonely when you are having fun! There are tons of sites out there that have calendars of activities in the area and many of them are completely free. You can even talk some of your other mom friends into going with you.
But despite all efforts, I still have days where I am just lonely. I think that is normal. There is something to be said for adult companionship and as much as we love our children, it just isn't the same. I think I would probably be just as lonely if I lived by myself and didn't have a child. Just hang in there, stick to your plan, and you will get past these moments.
I hope this helps. Good luck!
B.