Honestly the tiny bit of blood in the stool wouldn't concern me either if it was just the one time. Dogs sometimes get blood in their stool if they aren't feeling well and its not always a red alert to go to the vet (as a side note, if a cat has blood in its stool that's definitely a sign to go to the vet).
Now on to the gagging and bile. Is she regurgitating or vomiting? Does the bile just come up as if she burped or does it come from her stomach heaving and retching? Do you notice a pattern to this gagging? Is it at a certain time of day? What's the time frame between food and gagging up bile? Is there ever any bit of food in the bile or is it just foamy yellow/white stuff?
Since she's been eating the food since April that wouldn't be the first thing i'd guess was the cause. Has she eaten anything else recently? table scraps, new treats, something outside, poop? Are you using any new cleaning products, laundry detergent, air fresheners? Have there been any changes within the household? Has anything happened to your other dog (s)?
Personally if I was in a similar situation with my dog I'd be watching every little thing that she does. note every time she regurges or vomits and what it is, every time she urinates and deficates and the consistency of it, when she drinks and how much she drinks, and when she eats and how much. Since she is so small you don't want her to regurge/vomit +/- have diarrhea multiple times because she can dehydrate and get very sick very quickly.
Everyone is going to tell you to be safe and take her to the vet because with these little guys you just never know what's going to happen. We are not there to see the situation ourselves so we can only go by what you are telling us, and then tell you what we'd do. You are the one with her and know her best. However, i'm going to assume your other dogs are bigger and that Lulu is your first tiny dog. Just know that while you may be able to wait it out with a bigger dog, its not a good idea to do so with a tiny toy breed.
good luck with Lulu
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