It's entirely possible that you have CMV, and transmitted it to the baby while you were pregnant. So if you test positive for CMV, I wouldn't necessarily let anyone claim that it had to be the breastmilk. CMV sheds in breastmilk, but the CDC here in America does not recommend against breastfeeding in CMV positive moms. Usually infections via breastmilk transmission are asymptomatic for infants, so they just don't worry about it. And at this point, what's done is done anyway--once baby is exposed to CMV, they can't get "unexposed," so there is nothing at all to gain from withholding your breastmilk.

I'm not sure if the statistics are any different in Sweden, but here in the US, something like 50-80% of adults are infected with CMV by age 40. It's very, very common.