Little Misty is very lethargic this week.

She's eating and drinking and pooping fine, but just isn't her usual self.
Am trying to figure out why, here's the two things that I've come up with:
I gave her her heartworm treatment on Sunday, could that make her feel lousy? She's on Heartguard Plus chewables.
We recently sent off her foster brother to his new home, he had been here nearly 6 months, is she feeling sad and lonely?
We never treat the lawn with anything, but the bugs and the weeds have gotten so out of hand that my husband decided he had to do one treatment this summer and he treated with a turf builder and also with a Grubex Plus. We watered the daylights out of the lawn before allowing her back in the yard, but I'm worried that wasn't enough.
Poor thing, I hate to see her so sluggish.