Its common to get dizzy and light headed after a massage, like it was mentioned, the toxins need to be flushed out with lots of water.
It can also be because you can be a little dehydrated after a massage, also because you are so relaxed.
Take it easy, plenty of fluids and hopefully you'll be feeling better soon!

Was the therapist qualified for pregnany massage? There are certain pressure points you should avoid when pregnant so hopefully its not a result of something like that