I can understand why you're hurt and having a hard time accepting this. If you haven't tried to talk to either of the couples, I'd suggest doing this first. They may not realize they're not including you in the mix, plus you'll never know the reason unless you ask.
Let them know how you've been feeling and hopefully they'll give you their honest answer. If they avoid answering or if they continue to leave you out after speaking with them, then I'd suggest to move on and focus on building new friendships.