Sounds like you have garden variety pruritus ani. Sounds fancy, but it's just Latin for itchy anus--and you already knew that! Everybody has pruritus ani sometimes and some people are plagued by it more frequently. Sometimes a specific cause can be found, such as yeast infection, anal fissure, or hemorrhoids. Other cases probably are related to hygiene; wiping carefully after defecation and showering/bathing daily with careful washing of the anal area often are the only required "treatment". Topical steroids, like hydrocortisone, often help--but sometimes can make things worse, especially if there is a yeast/fungal infection.
The only conceivable STD problems are herpes and HPV/warts. Herpes would be intermittent, not continuous; i.e. symptoms for several days, with no symptoms at all for several weeks or months, then recurrence. And HPV/warts rarely would persist this long, and undoubtedly you would have noticed the bumps on your anus. The chance of either of these is zero or close to it.
The bottom line (pun not really intended, but not avoided either!) is that you should see a health care provider. A general provider--family medicine, primary care internist, etc--is most appropriate. In the meantime, put STDs out of your mind.
Good luck-- HHH, MD
For reassurance about HPV, I should have added that warts cannot come and go over a week or two. The "bumps" might be areas of local tissue swelling due to inflammation.
Don't let embarrassment etc to delay making an appointment with your health care provider. It's a very common problem and you can be sure s/he has dealt with it quite frequently.
HHH, MD