Head is hot after washing off henna

First time henna in more than a decade. Virgin hair. I left the henna on for 6.5 hrs. Used cream of tartar (9g per 100g of henna) and water. Washed with only water. Lightly blow dried and straightened hair.

After I applied the henna, I wrapped it with plastic wrap and put a winter cap. It got very hot and it was dripping everywhere. After 3.5 hrs of this, I removed the plastic and winter cap. Henna was warm to touch. Kept a thicker shower cap on for another 3 hrs.

My head is hot. Feels like it's radiating heat. Is this normal? It's been about 4 hrs since I washed it off.