If not attending the game tonight, I would be vague in an explanation. Simply say something to the effect of... W, I won't be able to make S's game tonight. Had something come up that I need to take care of.

That tells her everything she needs to know, but nothing more.

Me 34, XF 27
Many years together
Son 4
Engaged
Not engaged
Many false starts by XF
7/16 new girlfriend comes into my life
2/17 girlfriend moves in my home