Hard to add anything to what said JSmooth and CharlesFinley, which I totally agree with!
One thing though... if she has a boyfriend, you wanna avoid sending "sweet" good night texts
!!! In the worst case scenario the boyfriend is there looking over her shoulder when she gets your message and that may seriously ruin her day. Even if it has not happened yet, she can legitimately fear it will eventually happen. In her situation such a message becomes intrusive and you are crossing the line my friend! You put her in a delicate position and I totally understand she backed off the next day, like saying "no more of this shit!"
Imagine yourself in her situation.. .would you like to get text messages from a girl you have only lightly flirted with while in the bed with your official girlfriend?
On top of that sending this kind of message to someone that is not your gf yet sounds like an AFC's stuff to me, but that is just my personal opinion...
If you have a chance, you have to work it out when the window is open, when it is your turn to game her... Not when she may be lying on the bed with her bf! If she is interested in you, she will send you signals when it is time to play...