I'd agree with a previous poster, sounds like reflux, most commonly caused by an milk protein allergy. The pain and explosive poo are classic symptoms. It's very common, my 3 babies have all had it. It's really tough, but you will get it sorted. If you want to continue breast feeding you'll have to go completely dairy free, check all your foods for hidden dairy. Alternatively have your gp prescribe a formula for milk protein allergy.
Oh and I'd go to a different GP, water down breast milk, seriously
the ignorance of some doctors is just shocking. Your GP should be able to help and refer you to a dieticians clinic.
Good luck