Amazon RDS running out of freeable memory. Should I be worried?

I have an Amazon RDS instance. Freeable Memory has been declining since setup over 1-2 weeks, starting from 15GB of memory down to about 250MB. As it has dropped this low in the last days, it has started to resemble a sawtooth pattern where Freeable Memory drops to this range (250 - 350MB) and then goes back up agin to 500 - 600MB in a sawtooth pattern.

There has not been any notable decline in application quality. However, I am worried that the DB will run out of memory and crash.

Is there a danger that the RDS instance will run out of memory? Is there some setting or parameter I should be looking at to determine if the instance is set up correctly? What is causing this sawtooth pattern?