The first season can be anything from 6 - 12 months of age, and then usually every six months or so unless you have them spayed. The signs are swollen vulva, although unless you know what she looked like before you may not notice any difference. She may bleed a little, or a lot depending on the bitch, and this stage usually lasts for about a week - ten days, with the total season lasting approximately three - four weeks.
After the blood stops showing bright red it turns a light straw colour, and this is the time she is most likely to allow a dog to mate her, and will try to escape to find a mate, so watch your doors carefully. If you stroke her she may arch her back and flick her tail to one side, which is the signal to the dog that she is willing to be mated.
Are you sure she is bleeding from her rear end, and has not torn a claw or cut her pad?