Spirulina is also a chelation agent so it removes heavy metals from the body. This can have all sorts of unintended side effects.
Additionally here is some info I pulled from wikipedia “Some spirulina supplements have been found to be contaminated with microcystins, albeit at levels below the limit set by the Oregon Health Department. Microcystins can cause gastrointestinal disturbances and, in the long term, liver cancer. The effects of chronic exposure to even very low levels of microcystins are of concern, because of the potential risk of cancer.”
So if you are having GI disturbances I would be concerned about the above statement in regards to the micrcystins.
Here’s another thing worth mentioning as well: “Heavy-metal contamination of spirulina supplements has also raised concern. The Chinese State Food and Drug Administration reported that lead, mercury, and arsenic contamination was widespread in spirulina supplements marketed in China.”