I'm sorry that your question wasn't answered in a timely manner. I'll follow up for my colleague, Dr. Bruce. To answer you directly, there are quite a few causes of hematochezia (blood in his stool or from his anus) but an adverse reaction to either Heartguard Plus or NexGuard wouldn't be one of them. In most cases, I'll find an anal sac impaction, sacculitis, or abscess. This is easily determined by having his vet express the anal sac glands. Supportive evidence of anal sac involvement includes "scooting" behavior, licking and chewing at the hind end, and the foul odor you've noticed. Other causes include:
Perianal fistula - German shepherds are predisposed
Neoplasia - more common in older dogs
Perianal hernia - swelling would be noted in the perineum
Anal stricture - diagnosed by digital palpation
Trauma - should be a history of such trauma
Proctitis (inflammation of the rectum and anus) - tenesmus (straining to defecate) would be likely
Colitis - a common cause of hematochezia
Coagulation disorder - an uncommon cause of hematochezia; the drugs you mentioned aren't known to cause coagulation defects
Rectal prolapse - would be evident upon examination.
Please respond with further questions or concerns if you wish.