Make sure you clean double thoroughly-cats have a very high sensitivity for tasting/smelling-yes try something citrusy-some cats actually don't mind citrus stuff but some don't like it at all
Is her tray big enough and if it's open she may want more privacy-especially as she now has kitts-she may feel her babies are at risk and wants to feel safer-and telling you by leavin' prezzies is the only way she feels she can tell you-usually when a cat does this it can be for various reasons but usually it's because they're unhappy about something and trying to get our attention-if she's mixing still with her other fur siblings their maybe something going on that isn't obvious to us
