If you're going to attempt anal, then I strongly suggest you educate yourself on how to do it properly first.
If you do anal right, then it should never hurt. You have to take it very slowly, especially if it is her first time. ALWAYS use lube. Start with one finger and just press it against her asshole to get her ass used to something getting ready to enter it. Then, slowly insert the tip of your finger. Let her get used to the tip being in there, before you try to push in any further. Then try to go in up to your first knuckle...
There is a really good video guide to anal sex. Just do a google search for "Expert Guide To Anal Sex." It explains everything perfectly. It's basically a porn that explains anal sex, so don't watch it at work
