Betta Attitude Change

[phantomlink]

my girlfriend has a betta in a hexagon tank and its been fine for awhile. Out of nowhere about two days ago it went from small bubbles on the top of the tank to this massive nest in a corner. When she looks at him he flares up, and when she puts her finger in the tank he used to swim away but now he comes right at her and flares up. Is he just going through a mating phase but theres no females and it'll go away or will he just be mad from now on We've had him for about a month and he was fine by himself and even in a tank with Plattys once he got used to them and they stopped swarming him