What is this a mistranlation of?
There is a Chinese idiom 心如鹿撞 that has " deer" and " heart" in the same phrase. It means "like a deer is ramming the heart" 

We use this phrase to describe someone's heart is pounding due to excitement or fear.

For example:  


When you are alone with someone you love in the dark. The anticipation and excitement would make your heart pounds (心如鹿撞)
When you are stopped in traffic by a police officer, with a dead body in your car trunk. You may try to act calm, but the anxiety and fear would make your heart pump like crazy(心如鹿撞)


As for the deer image and the phrase "you are in my heart" (implies a headline: "a deer is in my heart")--  It  should be followed by the subline: " My heart is pounding in excitement" instead of " lonely but afraid" (you don't have a dead body under your bed, do you?)