Spirulina is recommended as a protein supplement while fasting, particularly for hypo and hyperglycemics.
My bet is that you weren't reacting to the spirulina before, but rather the spirulina was causing you to detox some toxins that were causing the problems.
Before I got cleaned up I couldn't drink a glass of undiluted OJ, because my system was seriously overacid. Now I have no problem.
Detoxing causes many many changes, though sometimes symptoms get worse before they finally improve and then disappear.
Hang in, enjoy your spirulina and be sure to drink LOTS of pure water and keep your colon April Fresh every fasting day...