She will be 10 months on the 25th.... how do you tell if she is in heat? Oh!!! She is humping everything in sight. But nothing coming out of her daisy (blood).
Maltese are often very neat when they are in heat, and you might not see any bleeding. It sounds like this is what is going on though. Is her vulva swollen? If it is, she is probably in season. Watch it for the next few days, as it might just now be starting to swell. If this is the case, you might want to reschedule the spay, as it is more dangerous if they are in heat. One thing about dogs going into heat is that they begin to act strange a couple weeks prior to the actual heat. I can often tell when one of my girls is coming in because the other girls will ride her a couple weeks before.
I would definitely keep her away from all males until she is spayed. Ten months is about average for first heat. I've had some girls come in earlier. Also, some will have what is called a split heat. They can actually come in again within a short period of time.
And, for those who hold out for a female because they don't want to deal with the leg lifting. It's common with both sex.