An amnesiac woman and her son reconstruct the past 15 years of her life, unaware of a murder investigation closing in on her.

The first half of your logline is clear and concise (well done!), the second half is indeed a surprising turn - perhaps more allusion to their relationship might make your logline scan a little better? Otherwise, I think this would be very interesting to play with...

Agreed with DPG and Knightrider, good potential but the logline is suffering from structural problems.

The concept needs more clarification as to the start - incitng incident, and end - goal, of the plot.
Why MUST they figure this out now and what MUST they achieve by the end of the story?

Yep, the story does lack an inciting incident. Why are they doing this? Because she is a amnesiac, but then maybe phrase it like "When she suffers amnesia..." Also, I am not sure who the Protagonist is, the subplot of an investigation hints there may even be a Detective Character as well.

It isn't bad, in fact someone suffering from amnesia uncovering their past, only to discover they are a killer is a unique way to tell a whodunit story, and will tie into themes about what makes us who we are, do they start becoming the killer they once were the more they learn about the person they are etc.

Loglines are tough, but keep going with them. Heck if though I have done a few, I still get them wrong.