What limit are you playing? Many mid stakes or small stakes players make illogical calls all the time. Often calling to the river when they are likely drawing dead. Someone could be calling your bet on a straight draw, a flush draw, overcards to the four (thinking your bluffing), a weaker ace, a mid-range pocket pair, or nothing at all.

Don't let irrational calls scare you away. If you are afraid of them, they beat you without even coming into play. Weak-tight is not a winning strategy in hold em.

That being said, you should be completely aggro either. You should find a balance where your plays mathematically have a positive expectation.