CONDOM SLIPPED AFTER PULLING OUT
Since you had a condom on during the intercourse itself, what you described was not really a risky situation.
As to your other questions, yes, if you had not been using a condom, it is possible for an HIV positive bottom to transmit the virus to an HIV negative top even though it is a much less risky behavior than the other way around. An HIV-negative bottom has about a 1 in 50 chance of getting HIV infected after one act of intercourse with an infected top.
As far as viral load, research has shown that the viral load in blood correlates to levels in semen and vaginal secretions. However, that correlation is only about 70% and it has not been proven what level of virus is needed for transmission. Even an undetectable viral load doesn't mean an infected person will not transmit HIV to an uninfected partner. Therefore it is very important that you continue to use condoms no matter your partner's viral load. In addition, a person's viral load can change, so that viral load a month ago may not be the viral load the day you had sex.
To your health,
HIV testing and care for those who are eligible -- at City Clinic.