google "homeostasis" now.
Anaemia represents a significant imbalance in the chemical composition of your blood. There are biological triggers that affect our behaviour and mental processes. Anything that makes you feel physically uncomfortable will usually result in a significant change, which can be called a symptom.
Iron deficiency results in a drastic loss of oxygen-carrying hemoglobin, which means the cells of your body, including your brain, are starved of oxygen and start to malfunction. This is why you were told that the tablets would raise your energy levels.
What happens if you hold yourself upside down for a long time, and then suddenly stand up straight? You lose your balance, right? This is called habituation, where you get used to unusual conditions and adapt in an attempt to make the best of a bad situation.
You have become habituated to iron deficiency, and the raising of your levels feels strange. Your discomfort is natural and will probably subside soon. If it doesn't, go back to your GP for a check up. There may be something else wrong, and the correcting of your iron has forced the symptoms of this other condition to the surface.