Hi there and thanks for your question.
There are many factors that lead to HIV transmission and the different risks associated with getting HIV.
In order for HIV to be transmitted, you need to basically consider 3 conditions:
1) There has to be a person with body fluids (blood, semen, vaginal or rectal fluids, breast milk) containing good quantity and quality of HIV
2) An activity with another person (unprotected sex, sharing drug injection equipment, pregnancy or breastfeeding, occupational exposure)
3) An entry point in to the other person (rectum, vagina, mouth, vein, broken skin).
If any of these conditions are not present, HIV will not be transmitted. Even if HIV gets into the body, it will not necessarily result in infection.
This website has some great information on HIV and transmission: https://www.catie.ca/en/pif/fall-2011/exposure-infection-biology-hiv-transmission
The long and short of it is that based on what you have described, your risk is very very low…that said, if you are in doubt…getting tested is the best thing. Which is what you have done.
4th generation HIV tests are the really really good tests for detecting HIV early on. The main reason is that these tests look both for a part of the HIV virus itself, and your immune system response.
I am not sure where you are writing from, but in BC, 4th generation tests are the first line blood tests done on people who have a blood test for HIV.
These tests can usually pick up infection by about ~2-3 weeks (16-18 days) after an exposure. At 6 weeks, 99% of individuals infected with HIV will have detectable antibodies.
37-38 days after an exposure should be plenty enough time to pick up an infection if it were there.
I hope this answers your questions.