What Causes Tinnitus to Increase in Volume (Pitch Change) Over a Period of Time?

My tinnitus will gradually/steadily get louder over a period of time. It was lower in the morning, but gradually increased to a high volume at 3pm. I did not eat lunch (had late breakfast) so food was not a factor.

If I lay my head on a pillow (not for sleep purposes), it will likewise increase gradually in about an hour (I can capture the pitch changes, sometimes another layer of sounds comes on).

My tinnitus gets louder when I lay my head down. If I tilt my head back and move my neck/head around, the volume and pitch changes. Not sure what causes this increase.