i need help from anyone who has pet mice

i bought two little baby mice yesterday they are really tiny but when i looked at them this morning one of them is missing a back leg im sure it had all four legs yesterday but theres no blood or anything like that and its quiet happy to wander about with out its back leg, and the other one seems to have a bit of a skin condition. they seemed fine yesterday and there still running about today but im really confused as to why one is now missing a leg the two mice are brothers so i dont think there fighting but i just dont no iv never had mice before.