We may need a little more info - was this a one-time event, or are you saying this happens every time you have sex? Also, when this happened, was it regular intercourse, or some other type of sexual activity (oral, manual, etc.)? And does it occur with ANY type of activity (Oral, solo, intercourse, etc.)? The reason I ask is, some types of activity may cause a more heightened response. At times I have noticed that my heart rate would go WAY up during intercourse right at & immediately after climax, and my heart rate & breathing were elevated for quite a while afterward. In fact, I stopped having sex late at night because I often remained very alert with my heart pounding & couldn't sleep, but masturbation or other types didn't seem to cause that. I should state that I'm in good health, work out, no heart problems, etc., so I don't believe the activity was too 'strenuous' for me. The only thing I could figure out was that this particular activity was more stimulating, exciting, or whatever. It may just be that this activity is extremely stimulating & you can't 'come down' after - ??
Of course, this is just a guess, can't tell from your post if it's just a feeling of being out of breath that you eventually recover from, or if there's something more going on. I would like to hear other opinions on this - I may be way off base, just trying to compare it to my own experiences.