She may have wished many people a happy Valentine's Day. It's nice that included you in her good wishes. It should make you smile, but don't ruin it by assuming too much.
A girl texted me Happy Valentine's Day. Could it mean something or am I over thinking this?
It means that she was thinking of you.
If you haven't received a card, then it's probably a group greeting text to all her friends. If you like her say something similar in return but no more.
Depends on how many other people she sent it to.