There was 'talk' of Robert Beltran being unhappy with how his character was underused on the show.
I don't know whether this is accurate or not, or how it translates into him having a relationship with 7.
There was a foreshadowing of this in 'Human-Error' though because 7 made a decision in the end to date Chakotay.
And the 'cargobay picnic' was apparently their 4th date.
So, they could have been dating for at least 2 to 4 weeks before Engdame commenced.
And when 7's cortical node experienced issues because of a too high emotional state, she said it would be inefficient to resume with the relationship (which she apparently decided to at least but on a back-burner).