It's supposed to be a very safe antibiotic.
Here is some info, including possible side effects, so you'll know what to watch out for:http://www.petplace.com/drug-library...lex/page1.aspx
(The Pill Book says side effects are rare at normal doses, but it lists gastrointestinal upset, including nausea, vomiting, loss of appetite and diarrhea as possible side effects. It also says that Cephalexin can cause fever and vomiting in cats.)
Good luck, I hope your kitty will do all right without any problems.