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 / Updated  / Source: TODAY Contributor
By Linda Carroll

The itchy red bumps came out of nowhere and it's unclear whether the mystery rash is something that needs a doctor or if it can be soothed away with over-the-counter salves.

That's what Chrissy Teigen is dealing with now. On Monday, the model and cookbook author shared a photo of her "thigh hives", a rash that apparently also spread to her bottom.

Teigen didn't say what caused the skin redness or whether it itches, but contact dermatitis can be a common allergic reaction to skin care products. As it turns out, plenty of rashes respond well to over-the-counter remedies.

But some are a sign of something more serious, experts say.

Related: How to treat dry hands, cracked lips, itchy arms and other winter skin problems

One important rule of thumb is to seek help if the rash came with other symptoms.

“The majority of rashes are not life threatening,” said Dr. Daniela Kroshinsky, an associate professor of dermatology at Harvard Medical School and director of inpatient dermatology and pediatric dermatology at Massachusetts General Hospital.

“But, if you’re feeling generally unwell and having a rash, that would be a reason to seek medical attention Your primary care physician or dermatologist should be able to help you triage whether it’s something that should be seen and whether it needs to be seen urgently.”

A rash can be a warning sign of a serious medical problem, such as an autoimmune disease or a Lyme infection, explained Dr. Laura Ferris, an associate professor of dermatology at the University of Pittsburgh Medical Center. Or it could simply be the result of a harmless, though vexing, insect bite or a brush past some poison ivy.

Signs that your rash needs to be looked at by a medical professional:

1. If you have a fever or pain accompanying the rash.

You should get it checked out, Kroshinsky said. It could be a sign that you have an infection or are experiencing an allergic reaction.

2. If you have a sudden spreading of bruise-like lesions.

It might be a symptom of vasculitis and you need to get that looked at because your clotting cells might not be working right, Kroshinsky explained.

3. If your rash continues unabated.

You might have an infection, Kroshinsky said. “Some rashes start out completely benign, but then a secondary infection develops because the integrity of the skin, which is a barrier against potential pathogens, has been disrupted,” she explained. “Signs of an infection include warmth and pain, yellow or green cloudy discharge and a bad odor.”

4. Any rash that is widespread.

It can be a sign a major allergic reaction. “For example, if this happens within two weeks of starting a new medication, the concern would be a reaction to the medication,” Ferris said.

5. Rashes that start to blister should send you straight to the doctor’s office, unless you have good reason to suspect you’ve come in contact with poison ivy, both experts noted.

6. Purple spots that appear on your hands and feet could be a sign of a bacterial infection of the heart, Kroshinsky said. “You can look at the skin as a window to the inside of the body,” she explains.

6. If you develop a circular shaped rash and you’re in an area where Lyme disease is endemic, you should get it checked out, Ferris said.

If you’re not ready to see your doctor, Kroshinsky suggested taking a photo of your rash.

“It can be helpful when you come in to see us since the rash could have changed by then,” she explained. “This will tell us what it looked like before you started treating it with bacitracin or hydrocortisone. It will help us get a sense of the evolution of the rash.”

Also, Kroshinsky counsels, keep in mind that the preparations you use to treat your skin condition might make things worse.

“One thing to keep in mind is that if you’re using a preparation with topical antibiotics, a significant number of people are allergic to bacitracin and neomycin,” she explains. “If the area you are treating becomes itchy or blistery you might want to stop using them and see a doctor.”

And if you feel unsure about the rash, don’t hesitate to seek medical advice from a dermatologist, Ferris said.

“We’re trained to distinguish between concerning and non-concerning rashes and getting you on the appropriate treatment,” she adds.