If she was on a perventative and negative she will be fine for a couple months. Depending when her last pill was. They dont magically get heartworms on day 31. They are protected much longer. If she is positive her not getting a pill isnt going to make it worst for a few weeks.
I would be more worried abou dehydration and nutrition or having her around other dogs till a full vet check. My rule of thumb if a stray waltz in on me is if they look and act totally healthy I ONLY GO TO THE VET for chip check.
I keep them seperated from my animals and look for owners. If it looks like they are with me a while then I do a vet check to get clearance they are safe for in my home and around my animals.
Since you have had her a while I would get with twylas about fostering her so you can get some help with a vet checkup and make sure all is well with her health. 4951