There could be a number of reasons for your heart rate to be low, but yes, magnesium is calming and lower your heart rate. If you don’t have a heart problem, it’s unlikely to hurt you. My heart stays around 45-50 all the time and it’s fine.
If you’re concerned about it, check back with your pharmacy or talk to you r doctor. If constipation is a regular thing for you, perhaps you should talk to your doctor about that, as well since there are some medical conditions that can cause it. Diet/lack of fiber also causes constipation, so you might need to look at what you’re eating.