I would absolutely check it out with your pediatrician. It could be a growth spurt or something completely normal. Better to check than worry.
Our son had a period of lethargy when he was four, it was really worrysome to me, we struggled to put our finger on it for almost a year. It would come and go, I could tell something was wrong, several doctors assured me he was normal, even my husband finally told me to stop worrying about it. I finally figured out in his case it was food related. It turns out he has hypoglycemia and now that we know, we are careful to feed him according to the doctors recommendations and we have been fine for five years now.
My advice is that mom's know their babies... stick to your gut and if you are concerned, it never hurts anyone to check it out. Best of luck to you and your litte one T..