I am taking oral cephalexin daily 4 times a day . today i took plan b pill (emergency contraceptive pill) is it bad to have those pills interact?

Shouldn't be problem: It should not be a problem, as there should not be any interaction.

"Double trouble"?: Taking these two pills together will make one of them ineffective due to selective gastric absorption. It will usually be the Plan B that doesn't get absorbed and then it fails to work with a doubling of your person with a child and undesired pregnancy. Separate the two pills by at least 2 hours when taking the Plan B.