Can I get an infection and sensitive teeth using Invisalign?

I’m using Invisalign for 4 months now, I’m almost finishing. But today when I woke up I took my braces off and I saw in the back of the treatment a white fluid like pus, with a weird smell just in that area. I dont have pain at all I just had sensitive teeth since 2 months ago. I’ve never had sensitive teeth before and I brush my teeth very well after eating. What I can do?