Fatigue too quick

I was wondering. I've been training for a fight which is tomorrow for 9 weeks. I felt fit as anything and started my rest period. Then my opponent changed in which I thought it'd be a good idea to go for one session with my trainer to work on new stud for the new opponent. Within a minute or so of an intense pad round I started really aching and fatigued. It felt like it was my first training session. Now it's in my head that I'm not fit even though I've been through the nine weeks hard training. And worried about feeling like that tomorrow. Anyone know how I can go from feeling fit one day and getting tired so quick 3days later?