I received fellatio from a prostitute (worse decision ever, i know) without a condom and i am wondering what are the chances i contracted an std?

Oral sex: The is very little risk for stds in receptive oral sex. The only real possibilities are chlamydia and gonorrhea or herpes. That would be herpes type one, if you do not already have that common infection. Most sex workers take their health very seriously and test for stds regularly.

Very low: Hey man. This is an intrinsically low-risk activity as far as HIV transmission goes. It's also a great way to get gonorrhea, and there's a syphilis and herpes risk. These may not be the best people for you to associate with and you may have broken the law. But this is FAR from the worst decision you'll ever make. You gave and received love and you chose a basically safe option. Cherish a memory.