Guidolin had no complaints about his side coming away from the match with nothing, but believes that the scoreline flattered the hosts.
"Newcastle deserved to win, but the result is heavy for us because yes, Newcastle played better than us with more intensity, with focus, but in the second half, we had some situations, some possibilities to score and draw level, and maybe the match could have changed," he told reporters.
"But this is our job, it's my job. I know football. Maybe after three months of hard work to achieve where we are in the table, it's possible to have a bad day, and today for us was not a good day."
Swansea reached the 40-point mark in the Premier League with a 1-0 win over Chelsea last weekend, but Guidolin stressed that his side cannot afford to take their foot off the pedal just yet.
"No, I think no. We need some points for mathematical security to stay in the Premier League. It's not done yet, so we need some points. I don't know how many - one, two three - but it's important not to relax," he added.
"Football is particular and I have seen many surprises in my career, so it's important to be focused and be aware."
Swansea's last four games of the season see them take on Leicester City, Liverpool, West Ham United and Manchester City.