My once very happy 3 mth old suddenly cries constantly, won't nap, and won't feed easily. What could be wrong?

My 14 week old has always been a very happy baby who sleeps through the night (9pm-7pm) but has now stopped napping during the day for more than 20 minutes at a time. He acts like he's exhausted and won't eat when we try to feed him. We end up spending half an hour soothing him until he finally settles down and eats. (We feed him every 3 hours.) I just don't understand why suddenly he is so fussy and screaming in hysterics. Is he in pain? Teething at 3 months? What can I do to fix it?