I share your concern about your pet. How old is your dog? Does she have a history of prior illness? Is the lump on her back soft or firm? Has your veterinarian evaluated that yet?
Bloody mucus in the stool is caused by what is known as large bowel diarrhea or an inflammation of the colon. If the blood is bright red vs. dark red, this is confirmation that the irritation or inflammation is coming from the colon and not the stomach.
The condition is unlikely to be caused by a blockage since you have no symptoms of vomiting and your dog seems to be able to pass stool. But, it can very easily be caused by a dietary indiscretion (like eating wood). It can also be caused by Inflammatory Bowel Disease or a host of other issues like parasites (has your vet done a fecal exam?).
Her behavior change prior to the episodes seem to indicate that she has abdominal pain which is relieved by voiding the stool.
For starters, I would add fiber to your dog's diet. You can try canned pumpkin or metamucil (1 tsp/day) to see if that helps the situation. If it's not resolving, ask your vet to do a fecal exam. You can also try a home cooked diet for your pet to see if that improves the situation.
A good resource for home cooked diets for pets is Dr. Pitcairn's Complete Guide to Natural Health for Dogs and Cats, available at your library or book store.
In general, if your dog is not losing weight, vomiting or continually uncomfortable, this is not considered an emergency and you have the time to try a few things before you go back to your vet.
I hope that this is helpful. Please let me know if I can answer any questions.
The symptoms you describe are not related to heartworms. That is a respiratory condition, not a gastrointestinal one. Dark red blood is much more of a concern - it indicates bleeding somewhere higher up in the gastrointestinal tract. If she has intermittent vomiting, that is also a concern too. I would doubt that this is a bug bite of toxicity.
Here's what I would ask your vet to do:
1. Fecal exam - often parasites are the most common cause of vomiting and diarrhea in young dogs.
2. Change to a home made diet as recommended in the prior post. Cottage cheese and rice would work as well.
3. Blood panel - to look for cause of underlying disease.
4. Abdominal x-rays - to rule out foreign body or other problem.
I would NOT use additional antibiotics. There is no indication for this.
I hope that I've been helpful. If I have, please accept my answer. It helps us both!
Please let me know if you have additional questions.
Lisa Boyer, DVM