I lose my sound as i practice

I've been playing for a month now and when I start practising I really acquire a good tone but as I keep practicing I totally lose my sound. It takes about 30 minutes until I totally lose it. It feels like I forget how to play as I keep playing. I don't really know how to solve this problem. I end my practice just because when I blow I cannot get a sound as if it's my first day playing the flute. Anyone who has experienced the same thing? I'm open to your suggestions because it is so frustrating :(