12:21 PM PT -- Former federal prosecutor Michael Zeldin came on "TMZ Live" Tuesday to discuss why Stormy Daniels was being allowed to get into all these alleged details about her time with Donald Trump if it isn't exactly relevant to the crime that's been alleged ... and he had an answer for us.

Take a look ... Zeldin says prosecutors for this case want the jury to believe Stormy is credible -- and the only way to do that is to lay bare everything she says she knows. In other words, it's all about believability ... and her spilling the alleged tea will help in doing that.

Stormy Daniels is testifying in Donald Trump's hush money trial -- and she says he told her he and his wife, Melania Trump, don't really sleep together ... and this was back in 2006.

The porn star took the stand Tuesday in NYC, and she got into the nitty-gritty about her alleged encounter with DT back in the mid-2000s ... when she claims they banged, and which allegedly entailed some juicy details about his personal life.

One of the nuggets that came out during her time on the stand was the fact that she said, under oath, that Trump had told her at the time that he and Melania have separate rooms.

The reason this even came up is because Stormy says she was asking DT about Melania before they hooked up -- and she claims he told her not to worry about it ... allegedly following up by saying he and she don't even sleep in the same bed.

Not just that, but Stormy says Trump told her they actually have totally separate rooms.

Elsewhere in her testimony, Stormy said that during their alleged tryst ... she swatted Trump on the ass with a rolled-up magazine. Apparently, she reenacted this on the stand as well.

Other tidbits ... Stormy says that when Trump was chatting her up at the time, he was apparently very interested in the business aspects of the porn industry. She also says he asked her if she was clean/had STDs ... to which she replied she didn't.

Remember ... this whole trial is over the allegation that Trump and co. paid Stormy hush money to keep quiet about their alleged affair -- so, she's the star witness here, and it sounds like she's ready and willing to tell everything as she remembers it.

In terms of how Trump is reacting to all this alleged tea being spilled ... word is, there's a lot of glaring coming from his end of the table. He's front and center here ... talk about awk!