Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I'm sorry to hear about your girl's red blotches on her belly and increased licking and I understand your concern for her.
Do her lesions look like the link below:
If so this is likely a superficial infection of the skin called staph dermatitis.
It is caused by normal skin bacteria getting introduced under the skin by the dog scratching, licking or chewing at itchy skin. Itchy skin is normally secondary to allergies of some type.
By the time we see a rash they generally need oral antibiotics but there are some things we can try at home to help.
Clip the area around and over the rash very short and use a solution mix of Betadine solution and warm water, made to look like weak tea, and clean the affected areas twice daily. This is an antibacterial and will dry the lesions out to allow them to heal. It does stain horribly though so you probably want to do this outside or in a laundry sink. Another option is cleaning the area with chlorhexidene shampoo and warm water, then rinsing and patting the area dry.
Since she is licking the area then I recommend placing an Elizabethan collar on her as the more she licks the worse the rash will get.
If her rash isn't much better after 3 to 4 days of treatment she will likely need to see her veterinarian for oral antibiotics.
Long term to prevent the rash from returning we need to address the underlying cause. Allergies are the most common cause of itchy skin. I'll give you an allergy rundown of likely causes for skin troubles. She may have more than one allergy. Dogs that have one allergy often develop several with time. The effect of multiple allergies aren't additive, they actually compound one another.
Even if you don't see fleas I do recommend using protection. They are the most common allergen and it only takes one bite a month to make an allergic dog scratch so I recommend using flea prevention even if you never see one again. Frontline Plus, Advantage II or Advantix are excellent. Don't use over the counter products, especially Hartz or Sargents, as most are ineffective if not toxic
Other allergens can be inhaled (like grass pollen, dust mites or molds).
You can use a combination of antihistamines and high doses of omega-3 fatty acids to help with those (they also help with the symptoms of flea allergy). In combination fatty acids and antihistamines work synergistically, much better than either one alone. If her symptoms worsen seasonally I would think that inhaled allergens are a part of her problem.
You can try:
1)Benadryl (diphenhydramine only don't use the combination products with acetaminophen or decongestants as they can be toxic for dogs) at 1mg to 2mg per pound or one 25mg capsule per 15 to 25 pounds of body weight orally every 8 hours.
OR 2)Claritin (loratidine) at 5mg per 25 pounds of body weight dog once or twice daily.
OR 3)Hydroxyzine at 1mg per pound of body weight orally every 8 hours.
OR 4) Chlorpheniramine at 4mg to 8mg per dog once or twice daily.
Some dogs do better on one antihistamine rather than another. Pick one, give it a week trial and if it isn't working try another. Be aware that antihistamines can cause sleepiness or hyperactivity in some dogs. These side effects do wear off with repeated use.
Omega-3 fatty acids are fish oil products. 3V by DVM or Derm Caps ES are good brand name products. Use the high end of the dosing schedule for your pup's weight. I recommend a fatty acid dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give her 20mg of EPA per pound of body weight per day. For example a 60 pound dog could take 1200mg of EPA per day.
Antihistamines and omega-3 fatty acids work synergistically, better than either one alone.
Because she has a vaginal discharge if she is in heat her overzealous cleaning of herself may have traumatized her skin and led to the rash. If that's the case you should still put an e-collar on her, but then you will need to clean her vaginal discharge with a clean cloth with a dilute mix of antibacterial handsoap and warm water on it, then pat dry.
Please let me know if you have any further questions.