I don't think it's a demon. I think it's acidity or what you call heart burn. Heart burn can be caused by several things--not eating for long hours, eating too much, eating hot or spicy food, constipation and even stress.
My guess is, you decided to chant something to invoke a demon and then started stressing out about how to deal with the demon, if and when it arrived. The stress triggered off the acids, which caused heart burn.
My advice, don't dabble, lol