What Causes Rashes On Inner Thighs?

i hope you could help me. i have small red marks inbetween my legs and ocationally a rash appears down my inner thighs(but not all the time). I have no itching or burning sensations around the area, but my boyfriend at the time i had sex with mentioned that he had the same thing. I was wondering if this could just be stretch marks or something else i need to worry about?