Abdominal bloating is a common side effect of MiraLax. It should go away once you've stopped taking the MiraLax for a few days.
I tend to have constipation problems anyway and it's made worse by the medication I take. I've found that taking probiotics works best for me. They keep me regular without all the bloating and gas caused by laxatives. The one I take is called Jarro-Dophilus. Here's a link to where I get mine so you can read more about it if you're interested, but you can probably also get it at your local health food store: https://www.prohealth.com//shop/product.cfm/product__code/N0213