According to a 2012 Canadian Aids Treatment Information Exchange (CATIE) meta-analysis of various qualitative studies on the topic, the percent risk of contracting HIV from each instance of unprotected receptive anal sex is 1.4%, or one out of 71 exposures to a POZ top. That being said, though the risk is relatively low, it's entirely possible to contract the virus from a single exposure. Some factors may increase or decrease the risk, including a high/low viral load, concurrent STIs, possible tears/fissures of sexual orifices, and the use of PrEP. By no means is this meta-analysis entirely conclusive; the data was drawn as estimates from sero-disconcordant couples, some of whom may be lying, inaccurately remembering their sexual behavior, etc. All in all, don't faze yourself over this, you've probably not contracted HIV from a purely statistical standpoint, but do get yourself tested and medicate accordingly.
http://www.catie.ca/pif/summer-2012/putting-number-it-risk-exposure-hiv#bios