Being seriously immunosuppressed due to illness or drugs might, in theory, delay the time until an HIV test
becomes positive, but it has no known effect on susceptibility to HIV if exposed. In any case, azathioprine (Imuran) is a rather mild immunosuppressive drug.
Finally, it is the rare escort in the US or Western Europe who has HIV, so the chance you were exposed at all is low. Heterosexually transmitted HIV is very rare among commercial sex workers in Seattle.