i have been feeding milu nature's variety raw diet. she loves it!

in the beginning i bought her the beef, she got tired of that, so i bought her chicken, she did well on both the beef and the chicken. i gave her lamb yesterday...and she had very runny poop. is this normal? i don't understand why she would have runny poop from eating lamb? i also got her venison, but i was afraid to give that to her. am i doing something wrong? do i need to slowly introduce a new meat to her diet?
thank you!