Heylids... Tikka was around the high 6lb-low 7lb mark during her heat cycle. My vet told me it's very common for urinary problems especially during the first heat 1. because the dog doesn't clean herself/doesn't realize what's going on the first time and 2. because I didn't wash her enough... which makes me feel horrible since I bathed her each week as opposed to every 2. But it happens, this is my first female dog, and so I was bound to make a mistake. But you should get her looked at either way
A check up is cheap, and a urinary analysis runs 60-80 on top of that so really still not bad.
also Tikka didn't start bleening right away either,
like cap_noonie says... Tikka didn't bleed her first week, then she bleed for a good week, then it slowly faded away and her vulva went smaller/back to normal color.
Also the latest you should wait is 1yr. I too feel 6 months is too early, mind you I just got Tikka on Easter but we were going to wait until after her first heat cycle, because her hormones should all be in place and she should be mature enough in all other aspects that I would feel comfortable spaying. This is just from experience I noticed our male dogs that were neutered at 5months were much crazier and had behavioural problems vs. the male dogs we neutered at 1yr. But all dogs are diff. and yes, each owner has the preference they're comfortable with.
I look at it like this: if your son or daughter was going to have their tubes tied would you let them do it before they hit puberty? I mean puberty is the most important part in any stage of life in any creature... so to me, it makes sense.