I have had the same thing and in my case, I realized it was that I was straining the neck muscles (riding tense or forgetting to look around if I was pushing hard). I noticed it went away as season progressed (riding more naturally and neck muscles “in shape”). Now, wouldn’t you know, I get the same thing now that I’m trying to life weights, but it is from straining my neck the other way (down/out instead of up).
This is likely it if you're a road cyclist. Since you're keeping your upper body low, but you still need to see in front of you, you're forced to crane your neck back to keep your eyes pointed forward. This way you are probably building tension in your levator scapulae, rectus capitis, and all of the other intrinsic muscles of your posterior neck/back. Rectus capitis minor partly anchors into your meninges (protective covering of your brain/spinal cord), so it can often produce migraine-like headaches. If you notice any vision issues (lights/swirls/shimmers) prior to the onset of the pain, this may be a clue that it is neck tension.
I would stretch the back of your neck with some chin-tucks (see picture below. Make a double chin and hold it there for 10 sec -- you should feel the stretch at the base of our sull. Do multiple reps.) and "tit-looks" (turn your head a bit to the right and pull it straight down such that you're looking at your nipple. Helps to tuck your chin a bit -- you should feel the stretch radiating into the most medial aspect of your left shoulder blade. Hold for 30 sec. Look to the left and repeat. Do it 3-5 times per side.). These symptoms can also be signs of dehydration or cardiac issues, so if drinking water and stretching doesn't work after a few days, go see a doctor!
Here's a funny picture...
