You can help her if it happens again. But to be honest it may not. She's OK to be spayed up to a good old age, but preferably before 2 or 3. But a bitch shouldn't be spayed before her first season anyway.
But if it does start, you can try distracting her with increased play and walks. While she's distracted you can remove any toys that she might have adopted as surrogate babies.
If she goes off her food offer tasty treats like cooked chicken or mince.
If she starts nesting give her newspaper to shred
If she was in season in March she's not due again until September at the earliest so you have time enough to get her booked in. Ideally 3 months after the last season to allow hormone levels to go down, but can be later than that.
Have you tried any other vets? Mine have been doing operations for a month now.
If she's fit and well try not to worry too much. I don't usually have my bitches spayed until they are about 4 or 5, some have been later but there is a risk of mammary tumours if left later as far as I've been told.