That used to happen to me quite a bit when I took a sabbatical from my day job and started playing guitar all day long. I can only attribute my experience to fatigue and over training. I could feel that my fingers and hands were stiffening up.
What you are experiencing could very well be related to how much muscle fatigue your hands and fingers have been subjected to prior to the time you practice. For example if you use your fretting hand all day doing other task such as typing, driving, holding things (especially anything with more weight than a pencil) or texting - it's quite possible that by the end of the day your hands are just tired.
Also, if you practice too much without rest, you can stress out the muscles and they will protest by responding more sluggishly (even the entire next day). Tired muscles don't respond as efficiently as well rested ones do.
Certain mineral deficiencies can also contribute to muscle cramping and fatigue. I try to eat a banana or avocado every day to maintain potassium levels in my body and take a magnesium supplement.
Good hydration is very important. If you are using any type of diuretic substance (caffeine, energy drinks, nasal decongestants, prescription meds) all of your muscles (including the ones in your hands) will have a greater tendency to tighten up or cramp. Be sure you drink plenty of water all the time.
It is also a good idea to start every practice session with some gentle warm up and stretching exercises. End each practice session with some more stretching which will help dissipate any lactic acid build up in muscles that have been subjected to higher intensity use (such as when engaging in masterpiece shreds at blasting speed).
If your body is trying to tell you that it is tired, it's best to listen. If you find that your fingers just don't seem to want to work like they should, that might be a sign that you need to rest. And perhaps add the proper nutrients and hydration.
Hopefully if you follow some of my suggestions above - all of your practice sessions will find you playing masterpiece shreds with few mistakes.
Remember to keep it fun!