The abbreviated "I don't care" is more of an early warning sign in conversation. It's not necessarily guaranteed to be a bad thing if the person you're romantically interested in doesn't care about a certain item (what to eat, what time to go out, etc.), but it can be a major cause for concern if used in more serious situations (whether to see each other that night, running into an ex, etc.). One or two of these won't doom a budding relationship, but the more you start to see, the more problematic it should seem.
For those who are unfamiliar with it, this technically translates to "What're you doing?" But it often means significantly more than that. Generally speaking, this is often used as a three-letter way of asking if you're free enough to go see the person texting you. It doubles as a (slightly less desperate sounding) way to say "Hey, I'm bored and wondering what you're up to," which is generally more information than people are willing to admit in the age of screenshots and saved texts.
5. I miss you
There's two circumstances in which you'd probably receive this text. The first is from someone who you're currently seeing and just happen to be spending some time apart from. It's totally acceptable if you're gone on a business trip or away for a few days, but it's a little weird when he/she misses you while you're at work for the day or only separated for a matter of hours. As for the second situation, this is the classic desperation text from an (often inebriated) ex. In that case, it's best left unanswered, or possibly with a quick dismissal, unless you really want to go down that road again.