Some years ago, long before the DNA kits were a "thing", a man contacted my grandmother, saying that he was my grandfather's nephew - the son of a daughter who had been "given away" by my great-grandparents at birth, which was why no one in the (large, very close) family knew about her. Problem was, my grandfather had recently died and his gravestone had been erected into the grave he shares with my uncle, who died as an infant. We all figured this man who'd turned up on my grandmother's doorstep had seen the gravestone and thought she was a childless widow... after all, I said at the time, why hadn't he contacted my grandfather's older sister who lived in the next village over from his home in South Wales (my grandparents lived in the Midlands)?! So, he was sent away - by my grandfather's children, grandchildren, and outraged nephews and nieces - with several fleas in his ear.
Then the DNA kits became a "thing" - and I did one. Turned out that this man's own nephew and I were related... through a daughter my great-grandmother had given birth to a few years before she'd married my grandfather and his siblings' father. Luckily the nephew of the man was gracious and we've actually become friends - he's NC with the man because he is actually something of a small time crook (and he did con money out of my grandfather's youngest sister... and an awful lot of it; she paid for several holidays, an extension to his house and school fees for his children 
She had dementia and was thoroughly taken advantage of), and understood why we'd been so protective of my then recently bereaved, elderly grandmother.
So sometimes, shit happens - and you only find out through spitting into a tube and having your DNA sequences online. This unknown great-aunt of mine was also the product of an affair between my great-grandmother and the man who employed her as a housemaid/nanny for his then young children... and I know that if my great-grandfather had ever found out about it, @Claire14467 , he would have been furiously upset. I remember how I felt when my DC's father told me his girlfriend had just given birth to his baby - thus ending our relationship of almost 20 years. You have to process how you truly feel about what could well be true - your husband had sex with another woman, who conceived a child whilst you were pregnant. And then you have to work out if you can live with that fact. If you want to remain with your husband... or not. You have to discuss whether or not to inform your children that they potentially have a half-sibling (and perhaps more... not to be unkind, but there may be others; my ex has a son a year older than ours, which I discovered after I threw him out) and, if you do, support them through their confusion and possible anger towards their father (my daughter hasn't spoken to my ex since the day her half-sister was born, because she witnessed my immediate reaction to the information - and cannot forgive him for the pain it caused me). And if you tell them, and they decide to have a relationship with this young man... you'll have to process another sense of hurt and betrayal, plus guilt because they're your children and you want them to be happy!
Either way, you need to talk with your husband. He may well deny it, but DNA doesn't lie, so this young man will have to go through an independent test (paternity), with your husband's involvement, if you want the actual truth.
