This is not a normal behavior, but it also isn’t because of the presence of sperm. I saw a patient once that would do this post mating every time they tried to breed her. It would take a more knowledgeable internal med specialist than I to pinpoint the exact cause, and we may just not actually know, but you can rest assured that you are not it!
@YanchaOkami mentioned stomach twisting. To my knowledge, it’s impossible to do this by rotating your dog, even if you held her upside down. The reason this happens is because large, deep chested dogs ingest a lot of air while scarfing down their food, and this air/gas bubble causes their stomach to rotate. It’s a surgical emergency, but not one that can be brought on by sex.
The way I’d go about things is to monitor it, which it sounds like you are doing an excellent job of. If it continues, and there is no vomiting or signs of nausea and decreased appetite at any other times for your sweet girl, then there is unlikely to be an underlying problem and you should be able to manage it pretty well with the same general anti-nausea drugs like Maropitant citrate (Cerenia) that we prescribe orally all the time for car sick doggos. Instead of giving it before car rides, give it before sex.
Just to give you a few tools for when and if you should take her to the vet due to vomiting—always have to stay vigilant for pyrometra in our doggos—you will want to take her if she is dehydrated, or continues to vomit even after calming down/anti nausea medication.
There are two main methods to check for dehydration. The more accurate method is to feel the gums. They should feel wet and slippery normally. If they feel dry or sticky she is dehydrated. The second method is called the "skin tent test." This is performed by gently pulling up on the skin behind the shoulders and letting go when you feel resistance. Count how long it takes for the skin to return to a normal position. A normal skin tent is less than 2 seconds.
If it’s only at these specific times, and entirely dependent on sex, I couldn’t tell you the cause, only how to manage it. As long as she is still having a good time and enjoying herself, I wouldn’t worry too much.