I'd recommend you start from a basic diet (rice, beans, vegetables) for 10 days to clean the colon, and then start to re-introduce foods. Try them for 2-3 days and notice the effects. Then, you'll be able to scientifically prove if it's the mushrooms, or not!

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IF you dont mind self medicating , this helps me when ever i have this problem .

Try a course of antibiotics
Ampicillin 500mg 2 tabs /day for 3 days
It will fix it 100%

what the antibiotics will do is kill the excess bacteria in the intestines
it will also kill the friendly bacteria , so take some probiotics for a week after the course .

This wont prevent the problem from coming back after 15-20 days if you dont check your diet and eat healthy .

I wouldn't suggest killing off your colon bacteria with antibiotics, as your good flora is much more sensitive than the bad stuff- and you may not kill it, but just weaken your own immune system in the process. Instead, try starving off colon bacteria by cutting back on the carbs and fiber for a while, which they feed on.
For something similar, I was put on an almost all liquid diet for 3 days- eating only chicken noodle soup, soda crackers, and lots of fluids to kill the bacteria. Then keep eating mildly but introduce a fiber supplement (about another 3 days). This will also help if you have had any changes in stool.
You can gradually reintroduce your regular diet, and like honeybunny said, you will be able to better guess your trigger foods. Elimination diet can be frustrating (because you get sick of things like chicken noodle really fast!) but also the most effective to find any intolerance (such as gluten, etc)
Good luck and good health!