I have been on Cephalexin for 7 days. I am getting headaches that...

I have been on Cephalexin for 7 days. I am getting headaches that don't go away after taking Tylenol (I took 4 Tylenol this morning within 2 hours because headache was so bad and Tylenol did not help) and I am beyond tired- I am a normally very high energy person and for the past 2 days I literally can not keep my eyes open. Today at 12 noon I had to give in and take a nap (which I never do!) and woke up 4 hours later. Upon waking I could go back to sleep and sleep 12 more hours. I am considering stopping this med. I am taking it for a minor surgery that I had last week but I am exhausted!