The low memory warning is letting you know that your device is critically low on RAM. Make sure you close all other applications from the multi-task menu by following this procedure.

1) Save your work. 
2) Go back to your device's home screen.
3) Force quit the app from the background by either double clicking on the home button or sliding the device screen from the bottom and then finding the iForm App and swiping up (Do not delete the app).

4) Restart iForm by clicking on the app, and open the record you were previously working on.

The notification was added to the platform to let you know of this situation. This is not a bug, as it is a limitation of the hardware. Capturing media files requires more resources than a text field so you will encounter this more on older devices that have forms with lots of media.