is it possible you have a hemorrhoid? if you do, then that might be where the blood came from. If you've been constipated, it's very likely that it was not from your vagina, but that a blood vessel was strained when you were experiencing the constipation.
One thing you can, carefully, do to check for sure for blood in or around your cervix is to put a qtip alongside your finger and swirl around the cervix. The white qtip will tell you exactly what color the cervical discharge is, whether it's clear, white, yellow or red-tinged. Just be aware that the more you "play around with" your cervix, the more likely it is to bleed a little, as it becomes more friable and more easily irritated when you're pregnant.
You shouldn't be having constipation. I would caution you to check the label of the vitamins you're taking. If they have Ferrous Sulfate in them (FS) then that is the harshest form of Iron you could take and it causes wicked constipation. I took a prenatal with that in it, faithfully, each day with my first pregnancy and I had HORRIBLE constipation. I didn't poop for the last 3 or 4 days before my son was born, and then ended up pooping during delivery, embarrassing! and painful and uncomfortable.
You can get better iron from Heme sources like liver and red meats, spinach and other dark leafy veggies, legumes and whole grains.
And/or switch to a prenatal that has ferrous fumarate or glucinate or succinate in it. These are much easier to digest and cause little to no constipation.
My favorite prenatal is from NF, here's a link
I'm an apprentice midwife, although Im on hiatus right now, and I would recommend these to my clients. THey have a LOT of the herbs we recommend in pregnancy and the vitamins you need as well. They're vegetarian and quality prenatals, more opportunity to be digested well and used by the body rather than just peed out!
Hope that helps! I'm not endorsing the website I showed, btw, just one of many that sells NF prenatal forte.
I hope you get some rest and have no more constipation! Add more fiber, fiber caps if you have to, and drink more water! Avoid caffeine, as it's deyhdrating, and try to get rest! HOpefully that helps!!!