Alarm volume turning down by itself

I’ve realised in the past few days that every time I’m setting the alarm at night the ring tone volume is completely off and I haven’t touched it. Before it wasn’t changing, it was always on maximum. How comes now it change all the time if I don’t go in and I don’t change it? And I don’t connect my phone to any headset, I’ve just checked as I just wake up and as soon as the alarm went off I went to sounds and the volume was completely turned down. Any advice?