I have to say it does seem to be quite normal to be very sick after visiting the Dom.Repub.
I visited the country in 1999, 2000 and 2001 and returned home feeling right as rain afterwards, as did the three members of my family that I was travelling with. I would say that the vast majority of visitors to the Dominican Republic feel fine on their return to the UK.
I would say that this is quite normal. After all, the number of healthy holidaymakers must far outweigh the sick ones.