Is It Normal for a Premature Baby to Have Hard Stools?

My baby, a premature infant at 325 days, is now two months old and on a mixed feeding regimen. Before, she had bowel movements once or twice every one to two days. Four to five days ago, the frequency increased to two or three times a day. Three days ago, I accidentally gave her the wrong formula, which was meant for pregnant women. In the past two days, the bowel movements have increased to six or seven times a day. I want to know what the cause might be and how to resolve it.
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