Sporadic itching doesn't suggest either HIV or any other STD. However, there are two possible exceptions: Scabies (itch mite infestation) could cause itching, especially if the bumps you describe are where the itching is located, and if they are reddened. Scabies can be acquired by close association, like sharing beds -- and can be transmitted sexually. Pubic lice (crabs) are another possibility, if the itching is centered in the pubic hair area; if that's the problem, you should be able to see the eggs (nits) attached to the base of several pubic hairs and may be able to see the lice themselves, which on first glance look like small brown spots on the skin in the pubic area. If uncertain, or if the itching continues without obvious explanation, of course see a health care provider about it.
Anyway, none of this has anything to do with HIV, and it is extremely rare to catch HIV from a single episode of vaginal sex, especially so when your partner assures you she doesn't have it. But if you remain nervous, i.e. if my reassurance doesn't fully settle your fears, have an HIV test. You can expect negative results.
Good luck-- HHH, MD