Birds do not have periods. Signs of blood like this are always, always serious.
Birds don't have a lot of blood to spare and any loss can cause life threatening complications.
Fortunately your bird isn't at the bottom of the cage or acting 'off', but that can be deceiving too. They'll hide their problems until pretty far along, which makes most people think it happened 'all of a sudden'. Very rarely is this actually true. Birds are that good at it.
When a relatively new feather is broken or injured it can bleed. Since they're hollow, they act like a straw and cause a significant blood loss, so quick action is important.
You need to find the source of the bleeding.
First, with a clean cotton ball or soft cloth, apply pressure to the end of the feather while securely holding the bird and preventing any flapping or stress.
You can apply corn starch to the end as a sort of ‘stop bleed' product (safer for a bird than styptic powder). Regular flour will do in a pinch.
If after about a minute there is no change and the feather continues to bleed, it's going to need to be removed. You need to be calm and quick.
Using a pair of (preferably) needle nosed pliers or other slender tipped pliers, firmly grasp the bleeding feather as far down the shaft and close to where it's growing from as possible. Smoothly, in one move, pull the feather in the direction it's growing. Don't ‘pluck' it, but more ‘glide' it out.
This should stop the bleeding almost immediately, but apply some pressure for about a minute afterward to be sure. Then give your bird some quiet time to relax and for you to be able to observe. If bleeding starts again, use the corn starch and pressure to stop and be sure the blood feather is fully removed.
If your bird doesn't seem to recover soon or shows signs of weakness, loss of balance, lethargy, etc. - don't waste any time getting veterinary intervention. It's always better safe than sorry.
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Blood Feather
If, by chance, it's not a blood feather - you'll need to watch your companion all night - very carefully, to be sure there's no more bleeding from anywhere.
It would be a very good idea to have her seen tomorrow no matter what, ok?
Let me know how you make out.
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I'm glad to hear that you can't find a blood feather, perhaps she plucked it on her own or it's formed a scab inside the quill to stop the bleeding.
I'm also relieved to hear that her droppings are normal and behavior is overall fine.
Still, if any of my birds were to display symptoms like this it would be worth evaluating and running a blood test at the very least to check for anemia which is easy to result when there's blood loss like this.
Be careful about giving too much Gatorade. It's ok in a pinch if a bird is dehydrated or in a position to possibly become dehydrated - but in many formulas of the beverage there's added zinc. Zinc is toxic to birds.
Again, a sip now and then is probably not dangerous, but keep in mind that levels may be building up (the bird can seem fine for a long time and then become ill).
Also, not all types of Gatorade have the zinc, so read ingredients carefully.
Diluted, natural fruit juice is a nice treat (like 1/2 and 1/2 orange juice and water, grape juice and water, etc.).
Keep an eye on her tonight and make sure she doesn't start bleeding again. If she does, you've got the emergency options to stop it as outlined earlier, so be ready.
I hope you both have a good night!