My four year old dog started heavily panting, anxious with what appears to be a fast heart rate earlier this afternoon. It seemed to start suddenly. Now, he periodically awakes out of sleep and pants, but even during sleep, his breathing seems heavier than normal. I worry because I forgot to start his heartworm pills back up as soon as it was getting warmer outside. Is this an emergency, or can I safely wait to see how he is doing during the night?