What are my rights when neighbor drains her pool and floods my yard , possibly damaging the retaining wall between us?

The general rule of law is that a property owner may not use his or her property in a manner that damages a neighbor's property.  You may have a claim for monetary damages due to your neighbor's negligent discharge of her pool water if it caused damage to the wall or your landscaping.  Your damages would also include any construction costs reasonably necessary to shore up the retaining wall or repair the retaining wall if it is on your property.  Your neighbor may also be in breach of the CC&Rs if any.  You should consider reaching out to the HOA property manager to discuss your complaint to to have a clear understanding of which party or parties have the reponsibility to maintain the retaining wall.  The HOA may issue an order to your neighbor to stop draining the pool onto your property.   As a worst case sceario, if the wall is on your property and you have the  maintence responsibility for that wall and you believe your neighbor's actions and damage will continue, you could seek a restaining order.