
One of my girls had 3 phantoms, each lasting 6 or 7 weeks and what they all had in common was the weight gain and other symptoms were very early, on one occasion I had mated 2 bitches within 2 days of each other, comparing the 2 the one having the phantom was bigger by 4 weeks than the one who went on to have 3 pups.
I always treated her as if she was pregnant, started Panacur and puppy kibble at 6 weeks and stopped when she would return to normal literally over night.
She did eventually get pregnant and her weight gain etc was much slower.
So signs and symptoms are not always proof of a pregnancy, depending on the breed the pups will be vary in size and some carry them up in their rib cage early on making it hard to scan.
With her only being 12 months old she may not be mentally or physically mature enough to cope with a litter and you will have to hand rear them, so be prepared in advance.