Thanks for the additional information.
This sounds like an acute allergic reaction, although there are other possibilities (I’ll come back to that). So in humans we would call this hives. She has come into contact with an allergen. That is a compound that can cause an allergic reaction. The allergen could have been inhaled as with pollens, it could have been ingested or it may even be something she has simply physically contacted. Sometimes the cause is quite obvious but in many cases after the first incident we are none the wiser. It’s not unusual for an individual dog to react in a similar way more than once over time and often when this happens an owner can see a common thread and work out the cause. So take note of anything you feel might be relevant.
Because she was outside and confined to a run a food allergy isn’t likely unless some sort of “critter” has been snapped up. Contact allergy is also unlikely in the run, that is usually plant species. So this may be an inhaled allergen and that can blow in from afar.
Severe case require cortisone given by injection and of course you would need to see a vet for that. But as long as the patient isn’t in any severe distress and as long as there is nothing to suggest that the reaction is affecting more important body systems (lungs, circulation, etc.) it is reasonable to be more conservative. Watch her breathing, if it becomes labored she should be taken to a vet. Pink gums are also a good sign. Severe reactions will usually surface within an hour.
You can use the antihistamine Benadryl Allergy (diphenhydramine) to settle down the reaction and make her feel more comfortable. Ensure it is not the combination product (Benadryl Allergy and Sinus). The dose rate is 1-2 mg/lb but do not exceed 40 mg total dose. Just a double check...she is on no other medications?
Do you have this medication on hand? It comes in a lot of different forms. If you are unsure of the dose, please tell me her weight and what the formulation strength is and I will advise you.
Now I said there were other possibilities. I’m pretty sure this will be allergic but occasionally we see an acute Staph dermatitis that can look like this. Usually we won’t see such a rapid development. Rottweilers do have a tendency to develop bacterial skin issues. With Staph problems there is usually an allergic component and anti-histamines or cortisone are still used but antibiotics may be needed too. If the “hives” start to weep or break open she will need to be assessed by a vet.
The Benadryl should settle things down within a few hours. Best to keep her inside away from the probable cause and where you can monitor her.
Please contact me back if I can assist further.