Why do I have headaches in the lower back of my head?

It feels more like a pressure than an ache, but it's uncomfortable. It's not from excercise because, um, i haven't been excercising at all lately. Lol. I did go out and walk around an outdoor mall today and it was very hot, but this didn't start till much later in the day, so i don't think they're connected. It also seems to travel from the lower back to my temples and back again randomly. Any help?