You need to call the vet now. If it sounds like a hypoglycemic episode they can give you emergency instructions until you can get the pup there. Ask them if you should give some nutri-stat or nutri-cal (if you have any) or they can help you go through and find something you do have in the house that may help. It may be something else or that your pup is hypo on top of another problem since you mention constipation as well. This may be an emergency. Give your vet a call now. Please let us know. I hope your baby feels better soon.