Sudden weakness

SugarAndSugar

So my friends and i usually do 1 muscle group a day every week except saturday and sunday. We usually do biceps and triceps on friday aka today. However when i was lifting i felt a gradual tension in my forearm and it was stiff in like 2 minutes. And right after i couldn't even do 1 rep of the weight i usually can do FOURTY reps of. Is this because of lack of muscle use or something else? Possibly injury?