My daughter, now 8-months-old, did the same thing at 7 months. I have a friend whose aunt is an infant specialist and she told me that it is perfectly normal for babies to go through a period of waking up at 7 months when they have been sleeping through the night. I guess it's called a touch point and has more to do with their development and reaching certain milestones as now is when they really start to gain a certain amount of independence. I didn't realize this and was feeding her those three times and then had to do sleep training to break the habit that I had helped her form.
As far as constipation goes, I had the same exact problem with my little girl. I didn't have to give her an enema, but instead started feeding her prunes, peaches, pears, oatmeal and lots of green vegetables. I eliminated (per the Dr.) rice, apples, carrots and bananas as those are constipating. Within two days of eliminating these from her diet, my little angel was feeling MUCH better and now poops anywhere from 2-4 times daily. Hope this is helpful.