I had a bill get sent to collections

So I had a baby in August 2019 and thought I paid everything. But yesterday almost a year later, I got a letter in the mail letting me know we have a balance and it has been sent to collections?!

Is that normal?

Will that affect our chances of being accepted for a mortgage even if our credit score is above 770?

Obviously I’ll pay it but I’m worried about two things:

1) if I pay it now, it’ll be a large payment on my bank statements that the lenders will see

2) if I don’t pay it now, won’t it be on my credit report when they run a credit check?