Mate, she's trying to give you a message without having to blatantly say it out loud! She wants you to stick something....anything!..up her arse. Maybe she wants you to finger fuck her arse while she sucks you off, maybe she actually wants anal sex. Maybe, she just likes the feeling of gentle caressing and fondling of the 'forbidden' zone as foreplay.
I tell you mate, just a suggestive, gentle caressing with a wet finger around the hole with the occassional tease over it, maybe even the occasional fingertip quick in and out, does wonders in working up a girl even if she's not into the full anal sex.
Try it. She'll love you for it.