What was that pain I felt in my forehead?

[Storker]

Fourthing the suggestion of a sinus squeeze. I get them occasionally on the tail end of a cold, even if my nose and ears have cleared up enough for me to think it's safe to go back in the water again. Sometimes I have to thumb the dive, sometimes I think I'm fixing it with a stronger-than-normal valsalva which usually ends up freaking out my buddy because I get a nosebleed. I'm slowly getting better at just thumbing the dive...

 

[uncfnp]

Fifth vote for sinus. If it was right over the eyebrow likely frontal sinus as devondiver diagramed. About 90 percent of the population have frontal sinuses. Then there are the oddball 10% without frontal sinus cavities, affectionally known as "hard heads" (yep, I'm one).

But I have also briefly felt the pins and needles sensation on my exposed face when passing through a patch of those stingy no see ums.

 

[The Chairman]

Open up the passages and use saline to wash through.

This is so important if you suffer from allergies/pollution. Flush yourself a few times a day with something like this:

If you're on the dive and feeling discomfort, go ahead and do snot rockets on the boat. Just aim for the ocean, but get that mucous out.

 

[pafindr]

Or it could have been a mask squeeze too.
Since you said you were dropping fast and the pain was below the eyebrow I'm guessing you weren't equalizing the air space in your mask.

 

[Storker]

If it was right over the eyebrow likely frontal sinus

Don't I know that feeling. The worst sinus squeeze I've ever experienced was not in the water, but on a plane. I just couldn't hold back the tears. Dang, that hurt a lot worse than any sinus or ear pain I've had underwater. And the pain was localized around that narrow spot between my eyebrows, stretching out under one of the eyebrows.