This is absolutely normal. All babies cry just before urination and while pooping. That's the only way a baby can express discomfort! An exclusively breastfed baby is not likely to get an infection. I suggest you omit the antibiotics prescribed. It is very normal that babies poop immediately when a feed is going on... That's a normal reflex.

It looks normal behaviour get a stool routine will tell you if there is infection if normal then no need of antibiotic