I'm allergic to penicillin. Was prescribed cephalexin for folliculitis. Stop taking it 3 days ago. Now I have hives all over. Delayed reaction?

Possible. A small % of people who are allergic to penicillin may also react to cephalosporin such as cephalexin. However, if your reaction did not occur until about 10 days after you started to take the medication, this may well be a true allergic reaction specific to cephalexin.