HELP! Potty Training...Regression or just Stubborn AF

My daughter has had trouble with popping on her panties. When she is home mainly. She is almost 4 and its been confirmed that she is rather constipated but she refuses ro drink enough water or eat any vegetables and she will only eat strawberries. I have to sneak veggies in spagetti sauce. She love mac and i have cut that out. Any advice of helping her with pooing? She was doing so well. She does have some speech issues but she knows how to say i have to use the potty. People are telling me i need to spank her. I dont parent that way though because i think its ineffective and causes a child to fear you. Any suggestions?