hmmm... i am having several thoughts about this.
On the one hand, the 'waste products' from your muscles might get released in your blood stream fast from a good massage, so your body has to work hard to process it. The system might induce nausea/sick feeling, so the body can put all its energy in processing instead of having you jump around and use vital energy.... (??)
Another option might be that the relaxed state (or leaving the usual survival mode that you are in) opens the possibility for your system to finally work on 'mending the damage done in the survival mode'.
On the other hand, somatic as in that someone tells you that you might feel sick, so you start feeling sick seems a bit unlikely. Most often it is the other way around, you tell the therapist that you felt sick afterwards and then he/she confirms it.
Somatic as in that the brain doesn't like your relaxed state as it opens up the possibility that emotions become exposed makes more sense to me... (??)
good question

I am not really sure what is happening....
... I forgot the possibility that your mind is really annoyed about you moving away from the relaxed situation. 'What the hell? I felt so cosy and comfortable and now you end it... Why?!'. The same mechanism that makes people become sick when they stop smoking or using other drugs. 'Where is my nicotine!!! I want alcohol right now!!!'. So in short, you are in the first stage of becoming a massage-addict
