No, it is not normal. Nobody would want to be a pescaterian if it were! :)
It is hard to tell what is causing your problems without knowing exactly what kinds of things you eat now vs. then and how much.
Also, it could not be food related at all and the timing is a coincidence. So if you have serious discomfort (sickness) it is best to see a doctor or two. Just in case it is something obvious and something that can be fixed easily.
Depending on how much and which type of fish you eat now, it is possible that you have to much mercury in your diet. This can definitely make you feel sick! If you think this might be a factor, definitely see a doctor.
Also, our bodies regulate themselves and the millions of different compounds in your body achieve a certain balance, when you always eat similarly etc. (everybody has a certain WAY of eating). It is not uncommon, that when you change something in your diet, make some significant change that the "balance" the body established does not work anymore. And you experience some "side effects".
But the body is pretty good (unless you have a disease) at finding a new balance, but it takes some time. So it is possible that it is nothing serious, just the a change. But it is always good to rule out the serious things first! And you should definitely not make changes simply because of this. Try to make them more gradually and give it some time. If it does not help, accept that this just does not work for you. Generally, don't to things when they don't make you feel better (after a while).
To give you an example, my family lives in Switzerland and when I visit them for a couple of weeks (eat their food) I always have some problems (nauseau, diarrhea, just feeling a little off). Even though it is the same food I ate 7 years ago when I lived with them, with no problems. I just eat very differently here in the US (my kind of food) and it throws by body off when I all of a sudden change what I eat.
Please contact me if you have more questions/concerns. You can contact me through my blog: http://www.health-foodie.com
Let me know if the problem got resolved or you saw a doctor. Hope this helped a little.
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