I don’t know. It is even possible that she was infected earlier but the possession came later. If it is just an infection the infected can behave mostly normal and is only influenced in specific situations. That could be compatible with true love.
It shouldn't shock, consider those we know were infected, and those that we suspect.
1] Elaine, on one hand unlikely, yet on the other hand she took refuge in the Summer Court
and what happened? She may have been infected by Justin.
2] Around the time that Elaine came to the Court, possibly before, but no doubt after, Aurora
became infected. Never proven, but she tried to take both Courts down before Harry and company
killed her. However up until the time she actually acted, she seemed very normal. Grieved, yes,
but I think Titania knows, that is why she whispered to Harry finally in Cold Days, "Nemeses."
3] She was acting a little power hungry anyway, so no one would have noticed that after she got
the Knife at Bianca's party that Lea was infected. However Lea, herself noticed it, told Mab and had herself committed to the ice cure. One that Mab, herself submitted to as a precaution seemingly.
4] Maeve, who's normal behavior was so outrageous anyway, that her own mother, Mad didn't
notice anything was amiss until it was too late, she could do nothing but order her daughter
euthanized.
5] Cat Sith, seemed normal, until Harry, now tipped off by Titania, realized that Cat's normal, wasn't and called him out just in time. Can you imagine what would have happened if he was on Demonreach with them when Maeve was getting ready to kill everyone?
6] Now Justine, yes, I think it is very possible that the "cure" for her mental illness wasn't the light feedings from Thomas, but Nemeses in stealth mode. And yes, very possible she could truly love Thomas, while the thing possessing her did it's deadly job. She'd appear "normal," sweet, intelligent, beautiful, Justine, who'd suspect her of anything? So yes, human Justine could truly love Thomas, meanwhile Nemeses quietly did it's deadly work.