Some budgie get marks and lose marks like that as they age so that mark is normal but you said its behaviour is different so there is always a possibility it could be sick, I suggest taking it to an avian vet and make sure everything is all good

That reddish spot came from nowhere above the cere! She also has started behaving strangely! Bites with hell lot of power! Also she's not that active as she used to be a month ago! :/

Should I take her to a vet?