If other strangers are rocking up, and you offered to pay, it sounds like they're being quite the R soles,.. but it may have to do with numbers, catering, and timing. eg when they knew the strange new GF would come.
If you were always going solo, then late in the day want to bring an extra, then that's a different ball game.
The question is, did they invite you and a plus one? The debate would then be, if not why not?
And should invites always offer anyone/everyone (singles) a plus one?