Hey
Providing you are both clean (you get my meaning?) I'm sure that you will come to no physical or mental harm as a consequence of having your arse crack and bud licked by your girlfriend. You no doubt will really enjoy this new sexual experience but if for some reason you don't, don't do it again unless you want to.
If you're really not comfortable with the idea of having your arse licked by your girlfriend don't do it and explain to her why. Sex should be a time when you and your girlfriend both enjoy and cement your loving relationship and so I don't think you should engage in any sexual activity you're not ready for because you won't enjoy it.
Finally may I say that arse licking for some is taboo and for others, me included, its just another sexual activity to try and experiment with. It's how you feel that counts.
Whatever you decide have a great time together!! -_-