Although panting can be common in dogs, especially when they are hot, I am not aware of it being a sign of being in heat. (No pun intended.)
I usually don't get concerned when my dog pants, but if the panting is long-term and the weather is not hot, I would be concerned.
When you asked this question, I looked it up on my favorite site and found a nice article that explains all types of possible medical problems and their symptoms...panting is listed in the article so Iam sending the link. Even though most of the article doesn't apply, it might be insightful:
http://www.veterinarypartner.com/Conten ... &C=0&A=150
I hope that it is nothing and that Lani is more comfortable today...please keep us posted.