What Causes My Kid Cry A Lot Before Going To Sleep?

My son is 32 months (turns 3 in November) and he has always been a great sleeper, usually going down for nap and bedtime with little to no fuss and falls asleep fairly quick. All of a sudden this past week he has been crying after we put him down and has stayed up fussing/whining/crying for 2-3 hours before finally falling asleep. He has also been staying awake during nap time (on some days). I really do not know what might have sparked this sudden change and would like to know if I should be concerned and/or advice on how to handle. My husband and I are trying to refrain from going into his room (although we have once or twice) - he is still sleeping in his crib at this point, we have not seen the need for transitioning to toddler bed just yet.