If it's bothering you, ask them about it. It's your life, you have a right to know. And it may be important at some point, that's the sort of thing you definitely want to know about before you, say, decide to have kids.
But I doubt they are trying to hide it from you to be mean in any way, they probably are worried that you'll feel embarrassed or singled out. There are lots of things that happen in early childhood that the kids don't remember, and the kids don't learn about until much later.
That doesn't mean there is something wrong with you in the head. Or at all. It means that as a child you had some sort of disorder which you have now gotten past/grown out of.
You're obviously able to function normally enough so you didn't know that you had this problem, so saying that there is something is wrong with you or you are broken is just...silly. You're not any more broken than you were before you overheard this conversation.
And it could be lots of things, not just autism, there are lots of disorders that you can have as a young child and then basically grow out of. Even things that happen during gestation and birth. Sometimes muscles and bones don't form right and have to be corrected, I know a kid right now who has some pretty severe motor and speech problems and has to be in a walker, have splints on his legs, etc, but as he's growing and doing pt and whatnot he's getting more and more 'normal' each time I see him. And the kid is sharp, too, just because he can't communicate super well doesn't mean there's nothing up there.
I didn't walk until I was 3, didn't talk until I was 5, and had lots of weird motor and processing disorders, but by the time I was maybe 7 or 8 I was pretty 'normal', just not super social.
My parents didn't tell me any of this until I was an adult, and when I was a kid they didn't seen any sort of help, deciding I was an 'indigo child', so I ended up struggling through parts of school. But also, given the time period, it's highly unlikely I would have ended up with a correct diagnosis, so who knows.
I finally ended up with a diagnosis of a high functioning autism-spectrum disorder, at 25 years old. Which explained a lot, but as far as my life goes it means...nothing.
I'm highly intelligent, married, I have a perfectly normal kid, I've held down jobs that required me to be social without a problem. Sometimes I don't notice or respond to body language if I'm not paying attention, but that's pretty much it.
And I've known several people that had physical or developmental disabilities when they were young, and now you can't tell at all, they are perfectly normal adults for all intents and purposes. So, don't let it worry you to much.
Maybe you are, but autism doesn't mean somethings wrong with you, its not like you're running around smearing shit on the wall. What you need is kindness, and an environment where you're allowed to flourish, and not feel like every step you make will be wrong, and criticised for it.
You might have autism, or you might have symptoms that correlate, like Sensory Processing Disorder, poor motor skills or delayed speech. It doesn't always have to amount to autism, and even if it does, you're not broken! You may find yourself relating to a group of people that you never knew about before, giving you a sense of belonging. This could be a blessing in disguise, but you'd have to ask them directly.
I have two friends with autism and they're beautiful people.