How long before first heat does puppy act strangely?

As per my vet’s advice, I am waiting until after my puppy’s first heat to spay her. She is ten months old and a small dog, so I have been told to expect it soon (if it hasn’t happened silently already — though I’m pretty vigilant about checking). She’s been acting strangely this week — sometimes sitting down and refusing to move on walks except when there’s another dog nearby, sometimes sniffing so intensely that she tries to run into the road to follow a scent (she’s normally a pretty light sniffer), not eating even when her food has exciting toppers — so I keep thinking she must be about to go into heat, but I’ve been checking regularly (the vet said to press a tissue against her vulva) and there’s still no sign of bleeding. Is it probable that the behaviour change is caused by going into heat? Or is there something else that could cause this shift?