Trifexis and time frame

I give my dogs Trifexis and they have been fine on it. Have been doing it since I go them. Callie has been on it now for almost 2 years now with no breaks . I'm am excellent about making sure they get it until this time.
I went to get it fro Sam's club because it was cheaper. It wasn't in stock but was told it will only take a day to get it in. No problem. That was a week ago and still no meds. I've gotten the run around about why it is not in. Hopefully today ??
Well this is the thing Callie date to take it was Wednesday (today if Monday). I think you are only allowed 2 days to be late, which I am not over by another 3 days. So looking at 5 days.
Do I need to get her retested for heartworm? It's winter time here, Indiana and been cold (30-40) for the past month. She has not been exposed to any other dog for several months out side her sissy who is up today on everything including her Trifexis.

So I'm just wondering if it's safe to give?

I've read a few sites that say it should be but that doesn't mean it's true.

Any thoughts? Thanks