I am currently using Truvada, Norvir and Kaletra as ART - I am undetectable-, as i am using Truvada, does it mean it works as PreP? do I have any risk to get re-infected if having sex without condom? can i infect someone else due to the same reason?
I understand that all of these new terms can be confusing! References to "ART" or "PrEP" or "undetectable" are relatively new for most, and still unknown to many. Let's understand them in more detail in order to answer your question.
"ART" stands for Anti-Retroviral Therapy. It refers to one or more drugs prescribed to someone living with HIV to extend the quality and quantity of their life. With the current ART's available, many people living with HIV are now able to get their viral load to undetectable in a relatively short period of time.
"Undetectable" is the term used to describe a viral load that can't be detected in the blood, and/or produces less than 200 copies. When someone living with HIV is undetectable they are considered to be healthy with a treatable condition (as opposed to a 'life threatening-disease'). When someone is undetectable for 6 months or more, they are humanly incapable of transmitting their HIV virus to others (http://www.thebody.com/content/77932/sex-58000-times-with-undetectable-partners--zero-h.html).
"PreP" stands for Pre-Exposure Prophylaxis, a regimen that an HIV negative person would use to stay HIV negative before a potential exposure to HIV. If someone is HIV negative, like undetectable, they cannot humanly transmit HIV to others. The reason this gets so confusing is because Truvada, as you pointed out, is the drug prescribed for "PrEP" while it is the same drug prescribed to many people living with HIV for "ART." The key difference is that an HIV negative person would use Truvada by itself for prevention, while a person living with HIV would Truvada in combination with other drugs for treatment.
So back to your question: When you are using your meds and maintaining an undetectable viral load then you are actively preventing HIV as well. But the medical term for this would not be "PrEP", it would be referred to as "TasP" = Treatment as Prevention. Both PrEP and TasP are tools that have significantly reduced HIV transmissions globally. But the former is used by an individual without HIV, the latter by someone who does live with HIV.
As far as reinfection (or what is sometimes called "superinfection"), it is considered extremely rare. When you are taking ART and undetectable, your small chances of being reinfected are significantly lowered. (https://wwwn.cdc.gov/hivrisk/what_is/hiv_superinfection.html). No one can tell you it couldn't happen, but we can say with medical certainty is extremely unlikely.
And to your final question - as stated above, when you are undetectable for six months or longer, you are not able to transmit HIV or infect another person.
To learn more about being undetectable, and to read more about the science and politics behind the "U=U [undetectable=untransmittable]" movement, please visit https://www.preventionaccess.org/ .
I hope this information allows you to enjoy the kind of intimacy you prefer and assists you with clarity and knowledge.