Girl, imagine this situation. You live in a country that consider even the smallest mistake punishable by death and the only precious thing u have left is ur 3 year old son, furthermore you've only been with the duke for a few month not even a year, would you fully trust that the duke wouldn't kill you or worse your son, thinking that you were deceiving him like a fool. She probably had to figure out a lot of things on her own, to decide on the right time, making sure her son is safe. She was also abused, degraded, sexually harassed and was treated like a rat by her own father who tried to sell her off, her self esteem has plummeted to the deepest end filled with doubt and fear for her son. If i was her i would had waited at least a year just to make sure that when i actually reveal it all my son would still be safe even if i have to die.
I don't know why the FL are so stubborn hiding the fact Nitense is his son, If the ML is asshole who only nice to the one they loved than I maybe understand, but so far ML is the one who reach out to her first, he is the best kind I assure you, many those ML in Isekai world would jump right into 'misunderstanding train' without trying to understand the FL and treat them less human and only treating the FL right after she changed into 'his type'. And he treated Nitense kindly even if he doesn't know he is his real son, a father figure indeed.
Don't tell me the reason why just because she want Nitense to live freely and not tied down to nobility, but that just so fucking Idiotic, in an era where people discriminate against other based on their 'noble status', no matter how talented you are but nobody would acknowledge you if you don't have a 'title' is not enough that Nitense is illegitimate and his mother refuse him to be adopted into the ML family, you want him to live as commoner instead? What a great mother you are, FYI you can't live freely without power and money. It seem the FL never thinking in a long term, well what do I expect from someone who planned to get pregnant with a stranger to humiliate her abusive and greedy father, what do you expect after you laugh at his face while revealing your big belly,he would welcome you with open arms huh, at least save some money and plan an escape route.