There are many causes of anal bleedinng and itching. Anal sex is inherently traumatic, probably especially the first few times--so just minor injury might be the cause, especially given the apparent vigor of the last event, as you describe below. You might have an anal fissure, a common problem in everyone, but more frequent in people who participate in receptive anal sex. However, you are right to be concerned about STD and you must immediately go get examined and tested: gonorrhea, chlamydia, syphilis and herpes all are possibilities. You need to be tested for HIV as well. (That doesn't mean I think you have it; I do not.) Your partner also should be checked, despite his previous negative test results. For example, 70% of gay men have genital or anal herpes and most have no symptoms; and other STDs often are asymptomatic as well and can be missed in routine tests. It would be best if you and he go together to the same provider.
And some general advice for you. First, establish health care with a clinic or doctor who has experience providing health care to gay men. You probably can find someone reliable by word of mouth among other gay men, especially if you live in a good sized town or city.
Second, I am nervous about your lack of condom use. If you are absolutely certain your partner and you will be permanently monogamous with one another, and if indeed both of you have been tested and negative for HIV and common STDs, in theory there is no risk. But at your age, most male-male partnerships are at high risk for other partnerships. At a minimum, you should speak openly and sensitively with your partner, and consider whether you should be using condoms for anal sex. Absolutely you must do that if either of you has other partners or admits to the potential for temptation in that regard.
Good luck-- HHH, MD
..just to add to my question.The intercourse was pretty aggressive.If it is a possible STD which one does it mostly sound like with these symptoms?