What happened is you dropped your calories too low.You went from eating 5,000 calories a day to eating 3,000 calories a day,in a short time too! That means you dropped 2,000 calories! Thats a catabolic diet right there bud.I don't care how much protein you're eating,dropping 2,000 calories like that will make you lose muscle.And I'm sure you weren't eating 2,000 above your maintenance level because that would mostly be all fat.Lets say your maintenance level is 4,000 calories(I'm sure you were only eating 500-1,000 above your maintenance level,right?).
So you started eating 5,000 calories.lets say during your bulk phase you gained 10 pounds of muscle.Since each pound of muscle burns 50 calories,your metabolism raised and you now burn 500 extra calories.That means your maintenance is 4,500 calories a day.So you went from eating 5,000 cal a day to eating 3,000 a day.That means to lose weight without any muscle loss you should,when dieting,consume around 4,000 calories -3,500 calories.
Try to eat 3,500 calories.If you still feel weak,then go up to 4,000.
Hope this helps,Poe.