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It could be a few different things. A heat cycle lasts about 3 weeks though in some dogs the heat cycle is longer and other shorter. There are also cases of dogs who have abnormal heat cycles where they might have some blood show, go into the second phase of heat and then back into the first stage of heat again. Let me give you a site that goes over abnormal heat cycles such as split Estrus.
Now it is possible that she did become pregnant and is experiencing a miscarriage or partial miscarriage. The problem is that until your dog is about 21 days or further along, it is almost impossible to determine if she is indeed pregnant. Your vet may be able to do an ultrasound now to determine if she is pregnant though waiting a little while longer would be better to see the pups and determine if they are viable. Dogs can lose one pup and still continue to carry the rest of the litter.
However, some older dogs can develop a uterine infection called pyometra. Pyometra generally occurs 60-70 days after a heat cycle. It is usually seen in older unspayed females but can be seen in any age. It is diagnosed by ultrasound showing fluid in the uterus. There are two types of Pyometra (open and closed cervix) Symptoms of open cervix Pyometra are a vaginal discharge that may be pus or bloody, lethargy, anorexia, increased thirst and urination, vomiting and diarrhea, and abdominal swelling though the dog remains fairly healthy due to the infection draining from the body. Closed cervix Pyometra symptoms are the same without much of a discharge, they appear very ill, there is usually weight loss and abdominal swelling and is much more extreme. Dehydration, shock and death can occur if not treated aggressively.
As you can see, this could be relatively harmless bleeding, or a partial miscarriage or the start of a serious infection. I would try and get her into the vet to determine if she is pregnant. An ultrasound can determine pregnancy as early as 21-28 days. He might also do a vaginal swab to determine if she is having an abnormal heat cycle.
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