Did you use a condom? If not then you had a risk if you penetrated, however no one here can tell if you did.
No one here can help you determine if you penetrated or not. If you feel like you had a risk, just take a 4th or 5th generation HIV test 28 days from now to know your status for sure.
What I have understood is that it's seemingly difficult to penetrate into an anus without application of lubricant. Nonetheless, I second CurfewX. Only you can decide whether you have penetrated or not. Therefore, please accept the advice offered to you.