Well, I always say never AP on an empty stomach. :P
In all seriousness, though, the stomach pain could have been simply doing magick on an empty stomach. The pain crossing your forehead sounds like a worn out third-eye, but I normally only hear about that when the AP is forced, not natural. I can say that I am almost certain no malevolent presence was attacking you, though. It doesn't match any MO. If you are worried, just make sure you have your room warded in case it happens again. AP is fun once you get used to it.