You're 18 days in. Are you giving each workout the care it deserves, or just going through the motions sometimes? Be honest. This is the time in a program when motivation flags, and workouts get missed, or mailed in.
Take a few extra minutes to warm up today if you find yourself struggling. Do some extra shoulder circles with small weight plates, or add an extra warm-up set. Do what's necessary to get ready for battle. Days like today are when it matters most.