I suspect that my 13 month old lab bitch is coming into her first season but am not sure, she seems a bit under the weather and a little bit off her food, is this normal, or could it be another problem, grateful for any advice.  

It can be. My Anna is always very subdued when coming into, and in, season. Others have not shown any effects.

Again, some go off work shortly before coming into season (Anna) and others will work right up to the day. After the season finishes some, again Anna will work literally right out of the box whereas others, my old Mandy for one would take a long time to get back into form. A broody 60 days was her mark.

As you can tell from this, there is no hard and fast rules. Each have their own little querks.

Regards, John

my bitch gets very naughty about 3 to weeks before, as does her sister. She just doesn't quite know how to settle herself, and will do things for attention. Once she is in season she is fine - don't suffer with not eating, but I do know others who go off their food. All down to each bitch.