The Austrian striker came on in the 84th minute at The New Den and headed in the winner in the 94th to secure the Reds’ first away victory of the season.
He told the Chronicle: “I felt ill after the game against Reading and I couldn’t sleep all night. I stayed in bed until Saturday morning (the day of the 5-3 win over QPR) then had a test but I felt tired and ill and the coach said ‘go back to bed and be ready for next week.’”
Schmidt’s first goal for the club was a consolation strike in a 4-2 defeat to Stoke City last month.
He said: “I was happy to get my first goal at home against Stoke but it was not a very important goal for our team. It still gave me confidence and it kept me going. I am very happy to get a winning goal in the last minute.
“I hope it gets me more gametime but I just want to help the team even if I am only on for five minutes. It’s a very good feeling for me and I hope I can score a lot more goals.”
Austrian football stops during the Christmas period but the Millwall game was the first of five in 16 games for Barnsley. Schmidt said: “It’s different but that is England. I like it. That is one of the reasons I wanted to come here because we always watch it like crazy on TV on Boxing Day.
“I always said it is my dream to play in those games even if it is a really big family time in Austria. I never was in a stadium on Boxing Day but it will be special and I am really looking forward to it.
“My family are visiting me and my girlfriend is here as well. Christmas feels like home in Barnsley.”