Well, looking at the MSDS for polyester resin and hardener (Methyl Ethyl Ketone Peroxide, or MEKP), I would only expect gastrointestinal pain if you had ingested resin or hardener. Since you did not mention eating resin, I'm going to say that your abdominal pain is unrelated to the resin. A caveat, though. I am not a doctor, if your symptoms worsen and/or do not improve, feel free to seek medical attention. BTW, I hope you were wearing a respirator.