The vet checked her out and said she looked fine but later tonight she started to dry heave. Is this serious? It didn't last long. About 30 seconds. But she has done it 3 times in the past hour and had it wake her up in her sleep

I would take her back to the vet. Did the vet just do an external checkup, or did he do more? Most likely only external if she wasn't showing any signs of trauma.

Now that she is showing signs of trauma it would be best to ask for x-rays to be done to make sure she doesn't have any internal damage.

Most times dogs are okay when they're stepped on, but the dry heaving, more then once, is something I'd be concerned about so shortly after she was stepped on by a full grown man.

Vomiting, panting and drooling are signs of pain.
Dogs often hide their illnesses and injuries very well, there could be a lot more wrong than the dog is showing.

With injuries like this there may be a small fracture or muscle damage where it doesn't disable the dog at first, but the swelling after wards is what causes the severe pain and discomfort.