Hello. This is Dr. Christian. I'd like to help.
How long has she had this problem and how often are the episodes occurring?
Ok. Thank you. Has she been an otherwise healthy dog? Does she take any medications?
How does she act afterward?
That definitely sounds like a seizure. At this age epilepsy is the most common cause. My worry is the same as your right now since she has had 2 tonight. Continued seizures can cause brain damage and overheat the body. If she has another tonight then take her to a veterinarian tonight rather than waiting. Quick treatment can prevent more. Epilepsy is the most common cause of seizures. Epilepsy is a chemical imbalance in the brain and occurs at a young age. Unfortunately there is no test to diagnose epilepsy. There are other possibilities such as infection, liver disease, and blood sugar problems so blood work is usually recommended to make sure none of these are the cause. Monitoring for more seizures may be all that is necessary. In severe cases medications are required. I definitely recommend having her checked by your vet tomorrow.
There are two types of seizures. Grand mal seizures are the ones most people know where they lose awareness and the whole body stiffens. Petite mal seizures are more mild and would fit what you are describing. That brings up a good point though. When an episode occurs make sure to video it so that you can show your vet exactly what is happening. That will help tremendously.
I don't think the grass is the cause but all the activity and excitement of being outside could lead to a seizure.
You're welcome. Good luck.