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This does not sound like a typical seizure. While it could have been very mild or focal (only involving part of the body -- typically the face), it would be an unusual presentation for a seizure. This could have been an episode of syncope. This basically means collapse and can be rooted in a heart arrythmia. My lab has episodes like this every so often. The the heart has this arrythmic period, they can stumble, collapse, have a dazed look etc. The best way to diagnose am arrythmia is a EKG, but the problem is recording the EKG while the episode is occurring. If another episode occurs, I would try to get video (cell phone cams are very handy for this). This can be very helpful to your vet.
Another less likely possibility is that this was related to a bulging disc in the neck or back. This can result in pressure on the spinal cord or spinal nerves which leave the cord thoroughout the body which can lead to episodes of nervousness, stumbling, etc. While I would not expect this to be the issue, it is slightly possible. My advice at this stage is trying to get video if another episode occurs (which may not happen at all). I hope this helps and gives you direction.
Heartworm disease could certainly cause heart issues, but this would not be what I would consider a typical presentation. So, it is possible, but not overly likely. Most of the time, these occur with no known underlying cause. You could certainly have bloodwork reviewed to make sure there is not other issue, but chances are things should be fine provided otherwise good health. I hope this helps.