If you have been camping/hiking etc. and drinking from a stream, just go get tested for Giardia.
If you think it might be MSG problem, find a couple Chinese places that advertise NO MSG and eat there and see what happens. If you have no problem, then go to a couple that don't advertise and compare results.
If you want to be hard core about self diagnosis, just write down everything you eat for a couple weeks, and when the explosions occur and see if you can correlate anything.
If you want to be really hard core about self diagnosis, look up something called an "elimination diet." Basically, to try to identify food sensitivities, you only eat 4 fairly tame foods (like rice) for the first week, then add in a different food or two every few days and see if one or more are triggers. I don't know the suggested foods or timing.
Also, if you drink a lot of coffee or soda with meals, it could be a caffeine thing---caffeine will speed you up. Try cutting that out.
Or , you can always go to a doctor....