Both my labs are small as well - 54 and 57#. Not sure that it is a component. A lot of the girls at the kennel I get my dogs from are all small (under 60#) not sure that there has been a particular problem with heat cycles and body size. Good luck with your girl.
P.S. thanks for the compliment on my girl - I of course think she is gorgeous as well. I am not biased or anything.