Hi again, Erica.
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Head pressing in cats may indicate damage to the nervous system, which could result from various causes, including prosencephalon disease (in which the forebrain and thalamus parts of the brain are damaged), or the ingestion of a toxic substance, leading to toxic poisoning.
Other possible causes may include a metabolic disorder, such as too much or too little sodium in the body's blood plasma called hyper or hyponatremia, respectively, a primary or secondary tumor located in the brain, or an infection of the nervous system, such as rabies or fungal infection. Additional causes may include head trauma (such as being hit by a vehicle), or from exposure to toxins, such as lead.
Symptoms may include compulsive circling and pacing, changes in trained behavior, seizures, damaged reflexes, and eye problems. Some of these symptoms may lead to lesions, for example, sores on the feet as a result of compulsive pacing, or injuries to the face/head as a result of pressing the head against a surface for long periods of time.
Tests may include looking at the back of the eye/retina, checking blood pressure, MRI and/or CT scan of the brain, and others, which you can discuss with your vet, if necessary.
Since you mentioned Pumpkin's behavior is not unusual in any way, hopefully she doesn't have any neurological problems, but it is always advisable to have her evaluated in person by the vet when you notice a behavior change such as this head pressing. She may just enjoy the feeling of the cool window against her head/face (if it is cool now, where you live).
I hope all will be well with Pumpkin!
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Warmest regards XXXXX XXXXX and Pumpkin,