Sometimes my friends say I'm forgetful, but I usually put it
    down to being trolled because I am sure the event or whatever
    didn't happen.

Obviously I don't know you or your friends but... at least in my personal social milieu - and those I've been witness to - this kind of "trolling" doesn't happen.

Do you have sibling or other close relative or friend who remembers your life with more clarity than you do?

If so, consider speaking with a medical professional.

    It would be interesting to know if my sleeping problems were
    connected to my potential forgetfulness.

So, even if not, wouldn't hurt to have a conversation.

They may not know shit about programming, but in my personal experience, they tend to know more than me about what's considered "normal."