When people have orgasms, it causes all sorts of muscular contractions. Some are obviously related to the sex organs, but many other muscles contract as well. For example, many people feel their toes curl when they have a strong orgasm. If your calves are tightening up more so than your other muscles, it may be that you just have a tendency towards such spasms. Or maybe you do a lot of standing on the job and your calf muscles are tired. As long as they are not cramping up so badly that you are in actual pain, I wouldn't worry about it. If you are getting actual cramps, then you might want to ask your doctor what you might do.