Melarsomine heartworm treatment shot

On 3/18, my chihuahua had his first heartworm treatment shot. I've only had him for about 10 mths & have hardly EVER heard him make a noise, must less bark /whine. Well since he's had his shot, he's whimpered about every single day. I've taken him back to the vet twice & she says she thinks it's his mouth & put him on antibiotics. He's done with those. She also upped his gabapentin which knocks him out then he's back up whimpering again. His 2nd/3rd shot, back to back, is coming up on April 20/21st &it really scares me since he seems to never gotten over the first one. I've read most dogs are back to their normal self within a few days after the shot. Well it's been 20 days for him. He whines soon as we get near him so I don't know if the whole vet experience has just scared him or he's really hurting. Just don't know what to do about his next couple shots being around the corner.

Hello!  I'm not surprised that you're worried - this sounds very uncomfortable for your chihuahua and stressful to you.  Obviously I can't see over the internet what is going on, but it reads to me as though your vet has examined your dog and does not think that the whimpering is to do with the Heartworm treatment but to do with his mouth.  Do you know why this is?  Did they find something on examination that suggested this to be the case?  There may be a very good reason for it e.g. a tooth changed on the same day, or that the stress of the vet visit intensified the way the chronic mouth pain is presenting (you may know yourself, that if you are generally stressed, a simple long-term pain such as teething can suddenly become less bearable).  However, your current instinct is that the pain was connected with the heartworm treatment.  I think a good first step would be to ask your vet to examine your dog again because if you think that your pup is in pain, treatment may be indicated and they should be assessed for this.   Second, you could ask whether you and they can read the data-sheet together, which is a formal slip of paper, required by law inside the box of medication, detailing the known side effects.  Best of luck and please let us know how you get on and what conclusions are reached.