Just added formula, poop got weird

I wasn't able to pump enough at work to feed LO (12 weeks old) sufficiently so our pediatrician suggested adding one 4oz bottle of formula a day. She gave me samples of Enfamil Infant (and I had some free samples of it at home). It's working out great for me but we are going on two weeks of it now and her poop has gotten weird. Before she used to go once or twice a day, the normal seedy yellow or green stuff. Now she will go two to three days before pooping out this thick, mud-like poop. She's blowing out diapers at daycare and I have to scrap the poop off her clothes instead of just rinsing it off. Is this normal when introducing formula or is this just not a good type for her? I'm thinking I should try another type but aren't sure where to go from here. Any advice?