This was a post I wrote some time ago which might help you understand what's happening.
The first thing to remember is that all bitches feel their seasons differently so making hard and fast rules of both identifying the start and the finish, but saying that, there are plenty of little signs if you know what you are looking for.
Starting at the beginning, if you are around males at all, they can often tip you off up to 6 weeks before by increased interest. Not wild interest but a bit more sniffing of her than usual. I’ve had bitches who get very slap happy in their working on the run up to seasons, not really got their mind on working, but others would work right up to the day! (But by the same token I’ve had bitches who will start working happily the day the season finishes but others who always finish up with a broody 60 days!) You often find bitches will start urinating more just before or during the season, in effect they are scent marking, a sort of, “Hi boys, I’m here!) Add to those symptoms, the vulva swells, usually a few days before the season begins. (Although very occasionally not until the season actually starts.)
OK, I know all that sounds a bit vague, all maybe’s, occasionally’s and sometimes, but such is the nature of ladies! What I’m saying is, look out for the subtle differences, particularly from around 6 months on. As soon as I start to get the feeling that the season is imminent I start on the “Toilet tissue test.” When a bitch is in season she starts loosing a bloody discharge from her vulva. In the initial stages there is not a lot of discharge, but when sleeping it collects inside, to drain out when she stands. So shortly after first standing after sleeping I give her vulva a wipe with a toilet tissue. This makes it so much easier to find. But remember, it does take a minute or so to drain out, so don’t be in too much of a hurry to wipe her.
The season it’s self lasts for approximately 21 days, although on rare occasions it can drag on for 28 days. The bloody discharge normally builds up during the first week, usually becoming paler and more watery during the second week and at this point the discharge may stop, or might continue for the entire time. But don’t be fooled! Even if it does stop she is still in season and is actually reaching her peak! In fact, if you were mating her then it is the 12th to 14th day when they are usually at their most receptive. Don’t be surprised if she is rather quiet during her season. It is not always the case but some can be very subdued. They often drink heavily. (Natural really when you remember they are loosing body fluids.) They also tend towards rather more urinating. It’s all perfectly natural and nothing to worry about.
During the season it’s safer to not take her out, and certainly NEVER off lead. If I do take mine out during the early part of the season I put them in the car and drive a way down the road to break the scent trail back to the house. I walk them on lead around a local industrial estate where I know there will be no other dogs. But I never take them out during the second week to the end of the season. In fact many bitches really don’t want to do much during that time so it’s no great hardship.
It’s never easy to determine the exact end of the season. It’s something which just gradually fades away. Even the swollen vulva does not go down straight away, and in fact never does return to the size it was before the first season started, she is a lady now, not a baby! For this reason it pays to be a little careful when you first go out after a season. I normally take mine out after 21 days, but take them at a time and to a place where I’m not likely to find other dogs. But after 28 days I relax and get back to normal.
So you see, it’s not an exact science. But don’t be frightened of it. The little signs will be there, it’s just that until it happens for the first time you will not really be sure which of the signs will apply to your dog. The thing you will find if you read some of the posts on here is that after the event most are saying that things were far easier than they thought.