Iphone text tone goes from quiet to loud on it's own?

I'm not entirely sure what to type into google so figured I would come here.
My text tone is on the quietest setting, so when it rings or if I get a text it's quiet enough, but I can still hear it.
Lately though, it keeps going off really loudly as if I've turned it up when I haven't. I press the down button to try and make that work… No different.
The only way to make it stop is by turning it off then back on.
Everything is up to date on my phone and itunes, all sorted, nothing has been done differently or added on to my phone.
If there are any suggestions or answers that would be wonderful!

So annoying