Erin,
That's not a common side effect of Milk Thistle. First check to see if the Milk Thistle is the problem. Try going off of it to see if the symptoms go away. IF the symptoms are still present, then it's not the MT. go back on it, then keep eliminating things one at a time until you find what's causing your reaction.
If it is MT, you might be allergic to it.
Also, are you taking Milk Thistle Seeds? How? capsules, extract, etc.