Should I apologize to my professor for not attending a lecture?
When you see her next, say something like


  I apologize for not showing up for lecture last week. I had a 
  doctor's appointment/seminar talk/committee meeting/friend's PhD 
  defense I had to attend.


Nothing more is needed than just to convey the message that you had a good reason to not attend class. Professors are busy people, they know that missing meetings is a fact of life. It only becomes awkward if you stayed away from class because you wanted to watch the last episode of Walking Dead/went out drinking with your friends/etc.