Most likely fertilizing the grass has given it a boost and they are eating much richer feed than they are used to.
You could try restricting their access to that grass, by strip grazing, having them in and feeding hay or straw to fill them up before they get to graze it.
If you're concerned about the bloating it would be worth treating those that look tight with vegetable oil and bicarbonate of soda to settle the gases.
Hope they feel better soon!