It is possibly the first sign she is starting to get bored in the relationship,maybe you aren't turning her on like you were . If she was really into you,everything you did in bed to her she would be into, or at worst she wouldn't object because she knows you like it. The fact she has said this suggest's all is not well,as she had no objections intially to you sticking your tongue up her a$$hole yes?And if it was a genuine issue for her,as it is for some girls, as some girls would be extremely self concious having their boyfriend kissing,licking where she takes a shyt , she would have said something back thenif that was the case . , and I'd wager if you stopped the ass eating, she would find something else to pick on about you.
I'd be wondering what is really bothering her to be honest.