Nothing wrong with you - same have happened with me. All goes well but then with a few moves all goes wrong (forks, hidden checks)
To be fair sometimes those forks happen because my opponent was just better. Sometimes I wonder wtf they're doing making silly random moves...next minute I realize they weren't random and it's too late to defend my king.
But sometimes I just forget everything and blunder every piece, and I find that enormously hard to deal with. Especially when I try my hardest to analyze why and get better, and still can't think and make giant mistakes. Oops hung my rook. Oops blundered my queen.
people have good and bad days, don't worry