Since daughter was 3 (7 now), she says she feels a pop inside her head behind ear with a small quick pain and then a warm sensation...Ideas?

Nasal allergy. This child may have nasal allergy and and a pop inside her head behind her ear may be due to air entering her middle ear after swallowing, similar to the feeling we have due to altitude change, may be associated by slight pain. These symptoms may resolve by allergy meds.
Sounds familiar. I can't say I have a great explanation but this is an accurate description of something I experienced as a child. On a practical level if this is not something that is associated with other symptoms and she is otherwise thriving you needn't worry about it.