Constipation could be due to a food allergy/intolerance. If it only occurs when eating soy and you have no other symptoms, you're probably just a bit intolerant to soy. In that case, I'd keep the amounts of soy in your diet low but continue to eat it if you like it.
If, you're also sneezing, getting nasal congestion . . . rashes, hives, etc. within 2-6 hours after eating soy, it's probably a full blown allergy and you need to avoid soy altogether. If you want to know for sure, an allergist can run the tests for you. But if you're short on money, do the elimination test to eliminate other foods. If soy always gives the same results, you have your answer.