Why do I get diarrhea after eating fish?

I like fish and have always been able to eat it all my life without a problem, until now. I can eat sushi, I can eat "fish and chip" fish. I can eat tuna sandwiches. I can eat lobster, I can eat prawns, I can eat oysters. These things don't effect me. What does effect me is fish from a nice restaurant, whether it's salmon or flat head or whatever, I end up feel sick an hour later. I have to race to the bathroom becuase my body just wants it gone. Afterwords I get the chills and feel like crap, like exhausted and drained. This rarely happens when I make fish at home, but now sometimes it does. The last time I got sick from fish was in the US, now I'm in Australia and the same thing happened. Why would I feel this way, what the heck is wrong with me? (I'm not pregnant, nor do I have any allergy that I know of, nor is the fish from the same restaurant every time.)