This is not unusual. There is a thin line between pain and pleasure, and for some people, that line is very blurred. I'm a good example. The stingibg heat of a good spanking is extremely exciting, and hugely sexually arousing. I love hard anal sex when my bum has taken a good hard spanking or caning. You lover obviously shares similar enjoyment.
There's nothing wrong with this if she enjoys it, so give her that pleasure she craves. She may have submissive traits that you have yet to explore. Learn to talk to her openly and honestly, and dont be afraid to help her enjoy her sexuality to the maximum extent possible. At the same time, tell her what really excites you too. The more you are open with each other, the more you will both enjoy!!!
Remember, there's no such thing as 'wrong' in sex, as long as those involved are agreeing to it.