I know it's too late for yesterday's get together, but in the future, one thing you can do is call the couple and explain to them just how down you are. Then give them the option of you still coming or not. I agree that not isolating yourself is the best thing for you, but I also understand just how it feels to not want to be a drain on someone else. I was amazed one day when a good friend invited me to lunch after church. I was a soggy mess from crying (had recently lost my Mom) and I said no, I don't think you want a crying person at your lunch table. Her answer "If that crying person is you, I sure do!" It amazed me, and confirmed just how much this friend loved me.

So give your friends the choice, you may be amazed, too.
And hang in there. Lamictal is good, but takes a while to get to a proper dose.