TL;DR: Completely out of character she (19) asked jokingly or not to marry her (I assume she just wants an upgrade to gf), and then asked if I was seeing anybody else. What would you respond to that?

So I woke up to a text from her (19) hinting that she wants to meet today (just to fuck because that's our frame) which I replied that I couldn't.

So after some unrelated shit, the texts were roughly like this (english is not my main language):

-OP: (joked with her with something totally unrelated and at this point I planned to bail, but the next texts throwed me off guard)

-Her: hahahaha

OP marry me

bride and ring emoji

-OP: haha I thought we were already married (I really thought we already joked about this with this girl but wasn't sure so throwed the next line)

At least in my mind we were

laughing face

-Her: face with hearts for eyes

Im serious! (obviously not actual marry but I think she wants an upgrade or is just shit testing me)

Are you seeing other girls? (this is what she meant but in my language this could be interpreted to "are you seeing someone?" so played with that in the next text)

-OP: Obviously

You ;) (tried to ignore it unsuccessfully)

-Her: two faces with hearts

Baby

You are my love

But I'm seriously asking you

And here is where I don't know what to say.

And is completely out of character for her, she has always been sexually flirty (the most flirty sexual texts I have seen in any girl, not explicit but for that reason I added flirty, is like she has very good text game and knows what to say to a guy) so I assumed she only wanted to be a fwb, plus she told me that she is sexually open, now I think she wants something more? I also think that the dread is working her hamster because I'm unavailable when she wants to meet and she actually thinks I will meet with another girl (I just said I have to do some things). At first I thought it was just a shit test (or still is?). So what would you say?