Okay, if your head gets very tired when you're doing the reflections, the cause of it might be that you are trying too hard. This is classic, especially in the breathing meditation. If somebody is focusing on the nose too hard they can create their own headache. So if you're finding that when you're doing the reflections your head's getting very tired it might be that you are pushing the reflection too much and you're not relaxed enough.
Now if for some other reason it's just something coming and going, then it could be ok to stop at that point and just observe what the sensation feels like, size, shape, etc. following the Unpleasant Physical Sensations technique. Then after a moment or two, you will probably be a bit more relaxed and you can go back to the reflections.
You might try to figure out where the tiredness is coming from. It might be that something in the reflection has brought up an emotional response in you, which might necessitate using another reflection to help that response change. For example, maybe you were reflecting on how fortunate you are and the thoughts keep going towards something that you saw on TV, maybe some dead bodies from an item on the news. Then you start thinking, "No, that's not part of the meditation". However these thoughts keep on coming, and coming creating stress in the mind, making you tired. It might be that you should let go of the how fortunate you are reflection and instead maybe reflect more on impermanence or wish the people in the news item some Compassion/Lovingkindness for a short while. That might relax the thoughts and then you can go back to the how fortunate you are reflection.