My oldest son has been having erections since he was an infant; he's nearly 7 now. His pediatrician (when he was an infant and she was doing a physical exam he was having one) said it's perfectly normal. But I haven't seen my second son having any, though I'm sure he's normal too. My second son became constipated when my husband (who eats large quantities of fresh fruit every morning) was gone for 4 months this year. My son would sit at breakfast with him and "share" his fruit, so while he was gone he wasn't eating as much, and then it hurt him once when he had a BM, so then he was holding it, which made it all worse. I took him to the doctor, who noticed a small hemorrhoid, and his little anus was very red and sore. He prescribed some miralax (I think) and some cream for his bum. I beefed up the fruit he was eating, and in time it cleared up. I only buy whole grain breads and such anyway, and really limit junk food, but fruits really help.
Also, "normal" is different for everyone in the BM department. For some, normal frequency is once a week. For others, normal is three times a day. My boys pooped five times a day in diapers, but now not nearly as often.