Adam, there are so many things that change our playing. Sleep, how much we drink, stress/peer pressure, intonation of the band, even if we haven't been to the toilet for a long while. If a cold is coming on, the histamines in your body also affect your playing in a serious way. My recipe for ALL students is a daily routine. This should be a minimum of 20 minutes and up to maybe an hour. These are exercizes that you play EVERY DAY FOR THE REST OF YOUR LIFE! Once you have had a routine for at least 6 months, THAT will tell you where you are on a particular day. That is the only real way to figure out what is going down. Even on a bad day, players with a good routine have enough stability to get through. The routine should consist of breathing, long tones, slurs and basic tonguing. After the routine comes the practice session. A routine is NOT a warm up!!!!!!