just include yourself! don’t wait for an invite
if that makes things awkward then there is probably something else going on.. in which case i would confront them about it, just nicely sit them both down, face to face (all together) and let them know what you have been noticing and if you have done anything wrong. it might be nothing and you could be over analyzing or there could be a problem
either way, you will find out once you confront them.