That sounds at least unusual.
There are several possibilities:
Either she is really reaching another heat period, this would be quite unusual but technically it all depends on hormones. As such: it is possible.
And depending on what she experienced since the last heat (like: constant sexual life, an infection, other female dogs in heat, hormonal imbalances and like 50 other reasons), it could as well develop out of another reason but "end" as a real heat period.
False pregnancy could as well lead to "heat"-like symptoms, but this should get checked immediately (ultrasonic pictures of her uterine tract and uterus is the minimum in this case, because: see below.).
An infection of the urogenital tract can lead to symptoms, which almost 1:1 "seem" like a heat. Swollen vulva, colored urine, discharge of fluids, even blood - that's possible. But this can as well end in a kidney or bladder infection or even pyometra and end deadly if the cervix closes up due to swelling, pus or discharges collecting inside.
Hormonal imbalances due to other health related issues or even stress could as well (but unlikely!) result in such symptoms or a new heat period. That's quite rare. Hormonal imbalances due to as example mammary tumors actually could inflict such changes, as the hormone balance can get thrown out of the acceptable value fields.
If I were you, I would (but this is no veterinary statement, as I stopped my vet studium to study technology, it's merely what I see as best practice):
- Check her for knots / tumors at the whole length of her mammary glands / milk strips by massaging those and feeling around if there's anything hard (movable or immovable) in the area, which has a higher hardness than fat and usual tissue.
- Check her vulva for any discharge which doesn't look like the initial stages of a heat, like pus or mixed color discharge.
- Massage her belly area (outside) and press in the middle area "between" the milk strips / mammary glands with two to three fingers - with some pressure, not "brutal", just as such as that her internal organs actually "feel" some of this pressure. If she shows any signs of pain without you actually hurting her, go to a veterinary and let her get a check (usually vaginal inspection or ultrasonic, nowadays mostly latter one combined with fever measuring).
- Watch her for other signs which are not normal or usual. Does she eat well? Do you both still have active sexual activities? Does she show any different emotional stance compared to her usual behavior?
- You can as well buy a fever themometer (digital one recommended) and measure her temperature @ anus (2-3 cm deep for correct tempeature). If it is between around 37,4°C to 39,1°C, that's the usual area. Around 39,4°C it's still somewhat acceptable, but shows that the temperature is slightly raised. All above starting from 39,5°C is fever, if it reaches 40°C or even above a veterinary visit is highly recommended.
As you already were on a veterinary and he told you that's a heat, this is a good "start", but.. I've seen quite a lot of veterinaries which didn't recognize early symptoms of more problematic infections. As such I recommend that you keep an eye on her symptoms, the discharge, any signs of unwell-being or fever, any signs of pain. And if the latter one happen, then visit a veterinary - probably best would be another one which doesn't think self-fulfilled: "Oh, she again. She's just in heat, I already told him this.." - that's worse than a neutral check through another person.
But any good veterinary will check her intensely(!) again if she shows any of the symptoms, even more if those get worse. And if you tell them what you checked for and how her symptoms differ from the usual norm, then they should take it as a clear sign that something isn't right and has to get checked thoroughly.