I'm not sure if any of this helps, but here it is. I will sometimes have increased heart beat, tingling head/light headedness, followed with a little anxiety, then nausea. But for some reason this only happens with certain therapists. I don't know why that is. One therapist just had this certain technique that kind of gave me an unsettling feeling. She was very good at what she did, but for some reason it did not jive with me. Or it jived so well that it delved farther then I was willing to go. Also, her office kind of freaked me out. I couldn't tell you why, but I just didn't feel comfortable there.
Has this happened to you with different therapist?
Also, I have to agree with Angie about the water thing. You might not be dealing with the toxins very well.
Are you getting massage on a regular basis, or are you just going when you hurt? I think the longer you wait in between the massages the harder it is for your body to recover. At least that is my own experience.
You should know, that your experience is not unusual. I worked on this one client one time, and she said she loved deep tissue, and I checked in with her often, but when she went home she actually threw up! She was one of those people that you come across, and you can just see the toxins dripping from their pores. So, I wasn't completely surprised.
My suggestion would be just like Angie said, and drink lots of water before and after, eat high water content foods, and don't eat 2 hours prior to your massage (kind of like if you were going to take a yoga class.) I also agree about telling your therapist about this, he/she might actually have some good ideas to help you too. Hope this helps, and sorry this is so long.