You don't mention if this is a potted plant or in the ground. In a pot, if it has good drainage, it can be watered every day. It needs the fluids to make the lemons juicy. If it is in the ground, water every other day. It may have too heavy of a fruit load for being a baby tree. It obviously can't support all the lemons it had on it so it is dropping those it can't support. Water more My lime has done the same thing, This year I have no fruit on it, last year it had a bumper crop.