X-ray showed that i was backed up with stool. taken miralax once a day for three days now with no result. does that indicate something serious?

Fecal impaction.: It seems like you have a severe type of constipation called fecal impaction. 3 days of Miralax is insufficient. Continue using Miralax to soften stool "from above", but also consider consulting with a GI specialist to get a prescription for an enema ("from below"). You may also need a different laxative. A combination of approaches is needed when stool is impacted. TTYD or use Healthtap Prime. GL