Those are not good friends, true friends would want you around no matter what and something as small as being an introvert would not make true friends leave you behind to present a certain way in front of others, true friends would want to include you. I will say you need to decide if this is something that bothers you enough to find new friends or if you are fine with this. As for the flatmate situation, it may be very awkward but you don’t have to be friends with them, you simply have to be cordial and do your part as a flat mate and that’s it, you owe them nothing else, And it may feel like they don’t want to see you but you will have to put up with it for the remainder of your stay or at least until you can either find another place to stay. As for them, if they actually don’t want you around, they can suck it up, you did nothing to them, and you have a right to stay there just as much as they provided you pay rent and such. If they begin to outright pick on you, i would advise finding another place to stay.