Does it have the appearance of being perfectly drilled? As in a small round opening? If so, it may be a predatory annelid that bores its way inside the clam, which looks fine for a bit, then suffers a rapid decline as it reacts to being eaten from the inside out. It would not take long for this to occur- so if there's no ill effect after a period of days/weeks since you've noticed, you may be in the clear....