Should I change formula?

My baby is 27days old and I am feeding her natureone premium formula. Her stool today is more form than other days and she did not have a bowel movement yesterday. She also appear to be straining a lot when passing this stool. Last week, there were 2 days of this type of stool but small amount followed by her soft mushy yellow stool. Otherwise, rest of the days stool is soft and mushy. Is this constipation normal? Also, should I consider changing her formula? My supply is low hence supplementing with formula.