Suddenly emojis cause text messages to be converted to MMS.

[AC Question]

I've had my LG Sunset for about 6 months and have texted using stock emojis that came with the phone every day since I got it. Today, I wrote a text and added an emoji - one I had just used in a prior text - and the text was suddenly converted to an MMS message. The text was a bit long - is that possibly why the emoji caused the change? It has never happened before. Any ideas on why would be appreciated. Thanks!