If you do a 5 day split, you will only be hitting that bodypart like 4 times a month. Does that seem like enough? No, it definitely isnt.
So, continue what you were doing; just make sure you replenish your body. There is no point in working out if you cant provide your body what it needs to build.
I have the same thing happen to me though. Sometimes, after 1 or 2 months, i will have a shitty workout. No good pump, tired etc. The next day my muscles will he wilted. What i do to combat this, is take 2 days off and just eat as much as you can. Carbs too! When you come back the next day, you will be good to go!