You are entering a whole new and different part of your life! Even though you're happy and excited about the baby, it's perfectly natural to also feel lonely for your 'old' life. For me, bring pregnant was a really contemplative time- you turn your focus inward more and more as the baby grows inside you- sometimes 'real' life seemed almost like a dream, lol!
If you have another child already, find other moms in the same boat! You know they're out there!
It's too bad your friends aren't doing more to stay in touch, but its pretty natural too. You'll make new friends, don't worry- and some of your old friends just might not know how to relate to you, or be unsure if you want to hang out.
Call up a couple of girlfriends and ask if they want to meet up for decaf lattes and tea instead of a drink. Or go to a movie or help you find some cute scarves to accessorize your pregnancy clothes.Or get a pedicure... Let them know you are pregnant- not contagious, lol!
But also realize that your hormones are all wacky right now and any little thing may upset you more than it normally would. When I was pregnant, my ex came home to find me sitting in the middle of the kitchen floor, crying, because we didn't have any milk to make macaroni and cheese, lol! He hightailed it over to the 7-11, but man did I hear about that later!
Everything that you feel right now is ok. Don't freak yourself out about feeling lonely- every pregnancy is different and you are a different person now than you were before. Think positive thoughts for your baby and try to get out more and be active. Good luck!