Why did my text messages turn from blue to green?

My girlfriend and I both have iPhones. Our messages have always been blue until recently. Now once they are sent they go from blue to green after a full 10 seconds or so, then instead of saying iMessage it says sent as a text message. Why is it doing this? It only does it some of the time. Is it something that she is doing on her end of the phone? Because even when a phone is turned off it still sends blue not green. She is having issues with her battery. Her phone turns off n on n the battery will say 85% and then almost immediately will read a much smaller battery amount and then shuts off. She turns it back on n it will read 85% again. It’s wired. However it shouldn’t effect my sending an iMessage or text message. Should it?

[Re-Titled by Moderator]