I have psoriasis and was told by a dermtologist to start taking probiotics. can taking them be the cause of recent boils on my upper, inner thighs?

Not likely unless:: Although infrequent, Probiotics can cause infection in patients with immune compromise, severe debility or chronic disease. (This has NOT occurred in a healthy individual.) Lactobacillus LGG, a common probiotic, has been associated with septicemia, endocarditis, liver abscess. S. boulardii, a common yeast probiotic, has been associated with fungemia(blood infections).

Unlikely: The bacteria that are considered "probiotics" are not the same as those that cause skin infections. They reside primarily in the digestive tract. It's most likely just a coincidence, but you need to seek medical care regardless of what's causing them. Best wishes.