Heartworm Meds & Seizures

Based on the data given about the timing of the seizures, it's not occurring at a regular time, the timing is not regular. The first time it was a week, the next time it was a little over a week, and the third time it was 3 weeks later. Correlation doesn't equal cause, and there isn't any correlation here. Clearly this doesn't correlate with any sort of regularly occurring event since this isn't at a regular interval, unless the original poster hasn't been giving the heartworm meds regularly each month. Idiopathic epilepsy, according to some quick research, is a known issue in pomeranians. Perhaps that's it, and the heartworm meds have absolutely nothing to do with it.

http://www.canine-epilepsy.net/index.html

That's the canine epilepsy network, which may be more helpful to you. 

This is a warning on the label of Heartguard Plus, and searching the Interceptor Website it said the same.  Of course not every dog has adverse reactions, but if you already have a dog causing problems, can you really feel safe giving them a pesticide monthly?

Merial, the makers of Heartgard Plus, cite vomiting and diarrhea as the most commonly observed adverse reactions noted in clinical field trials of the drugs. As the most common side effects, these reactions were noted in only 1.1 percent of cases where the drug was administered, according to the product's safety and administration guideline pamphlet. Additional side effects of Merial included in the pamphlet are depression, lethargy, unwillingness to eat, loss of coordination, convulsions, hypersalivation and excessive dilation of the pupils.