My Dog Has Heartworms But Can’t Be Treated Till Next Month! What Do I Do?

We just adopted a stray, 14 lb, 3 year old mutt (possible cockapoo) that just tested positive for heartworms. The good news is that it won’t cost NEAR as much as I thought, the bad news is, she is too skinny to be treated and we’ll have to wait a month.
She is very lethargic and sleeps so much of the time. Is this a symptom? Could she be in a later stage of heartworm disease? I can’t stand the thought of this sweet little girl’s heart full of adult worms. It’s killing me! How can I get her to gain weight quickly so we can do the test soon and I wonder if she’s in pain or if I can make her more comfortable.
~ Worried new dog mommy