<human>: Provide a brief overview of the text.

So I have a cat, a 2 year old female calico who is both fixed and up to date on her shots. I occasionally let her out at the evenings since she enjoys running around my yard. However, recently I have been noticing that there is this stray cat (I assume, as s/he has no collar) who looks about the same age as my cat hanging around my yard. My cat and him/her would sometimes play together. 

Well today I let her out as usual, and at first it seems like everything was going smoothly. But an hour later, I suddenly hear some snarling. I went to go check it out, and now my cat keeps growling at the stray cat, and chasing him/her away whenever s/he gets too close. I automatically brought my cat indoors, but that stray cat keeps sitting outside my front door yowling (which just makes my cat growl harder). 

Is there anyone who can help me explain what's going on?
<bot>: My cat is pissed off at a stray cat, but stray cat keeps meowing in front of my door for some reason.