I'm assuming the milk was frozen in a deep freezer? If not, it is only good for up to 3mos in a regular freezer I think. Either way it could be a problem, I would stop using it if you haven't already. And most definitely don't use the microwave to warm it up as it alters the properties of the milk. I don't know if that would cause the increased spit up, but better safe than sorry, kwim?
One of the reasons I wondered about reflux is being twins I'm assuming they were born a little prematurely, and it's very very common for premature babies to have reflux. But if it never seemed like a problem and they were thriving, then I would guess they didn't have reflux. However, reflux can still be present with the absence of spitting up so it still could have been present. Usually refluxy babies are in pain though, and you say they were always happy so that's a good thing.
What you could do if you want to know if a particular food is bothering her is go back to basics. Start offering only one food at a time for a few days, and keep a log of that. It should narrow down problem foods.
Otherwise, if she is doing ok I don't think there is cause for immediate worry. Keep an eye on the spitting up as well as her behaviour and sleeping, and definitely mention it at her next appointment.
